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david at istwok

Jul 27, 2012, 11:27 AM

Post #1 of 25 (1579 views)
Permalink
Recording directly to RAID
Is anyone recording directly to RAID? If so, what type/size, how many
simultaneous recordings and how well does it work?

I'm currently using 3 independent drives for recordings. The primary
reason to use independent drives has been to spread in-progress
recordings across multiple spindles. It's fairly common for me to
have 3 or 4 simultaneous recordings and 6 is not unheard of during
some busy parts ot the year.

While my drives currently aren't on death's door, they are a few years
old now and I have had a handful of scares in the last few monoths
where various drives (or controllers) have gone wonky. Fonrtunately,
the wonkyness has only been temporary and I haven't lost anything of
real importance. Still I'm spooked enough to seriously consider
adding some redundancy.

The two configurations I'm primarily considering are either a 4-disk
RAID6 array or a pair of 2-disk RAID1 mirrors. While the RAID6
configuration should be more reliable, in theory, I'm leaning toward
the RAID1 configuration. One big reason, and the reason for this
email, is the increased seek load on all drives with RAID6 when 4 or
more recordings are in-progress.

David
--
David Engel
david [at] istwok
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memmott at gmail

Jul 27, 2012, 11:56 AM

Post #2 of 25 (1544 views)
Permalink
Re: Recording directly to RAID [In reply to]
On Fri, Jul 27, 2012 at 2:27 PM, David Engel <david [at] istwok> wrote:

> Is anyone recording directly to RAID? If so, what type/size, how many
> simultaneous recordings and how well does it work?
>
> I'm currently using 3 independent drives for recordings. The primary
> reason to use independent drives has been to spread in-progress
> recordings across multiple spindles. It's fairly common for me to
> have 3 or 4 simultaneous recordings and 6 is not unheard of during
> some busy parts ot the year.
>
> While my drives currently aren't on death's door, they are a few years
> old now and I have had a handful of scares in the last few monoths
> where various drives (or controllers) have gone wonky. Fonrtunately,
> the wonkyness has only been temporary and I haven't lost anything of
> real importance. Still I'm spooked enough to seriously consider
> adding some redundancy.
>
> The two configurations I'm primarily considering are either a 4-disk
> RAID6 array or a pair of 2-disk RAID1 mirrors. While the RAID6
> configuration should be more reliable, in theory, I'm leaning toward
> the RAID1 configuration. One big reason, and the reason for this
> email, is the increased seek load on all drives with RAID6 when 4 or
> more recordings are in-progress.
>
> I have an Apple Xserve RAID array. Two RAID 5 volumes (one 500GB disks and
one 750GB disks, all IDE), each volume connected to my ESXi server via its
own fiber cable. I then have a 2TB LUN presented as one huge VMDK disk
image to my master backend, which is a VM.

Prior to this, I had a RAID 5 array of 200GB disks on a homemade OpenFiler
machine, presented to the then-physical MBE as an iSCSI volume (I don't
recommend this, when Myth suddenly loses its disk due to an awful network
setup, it gets angry).

Before that, I had an LVM volume set up of the same 200GB disks (This was
before recording groups existed), but I'm dumb when it comes to Linux and
accidentally deleted the entire volume.

In my current configuration, things get a bit choppy when I have 3
simultaneous recordings through my HDHomeRun Prime and I try to watch a
show on a FE, but this could be due to the VM layer of obfuscation. It's
not choppy every time and as I said, I'm pretty dumb with Linux so it could
be an issue with my setup. I'm on 0.24, and I hear that 0.25 has a much
lower IO load.

How many drives do you have? RAID 1 needs two disks, RAID 6 at least three.
I'm not sure how those are your only two options. If your disks are 2TB or
larger, I highly recommend RAID 6 over RAID 5. If you lose a disk the
rebuild on arrays that large takes so long that statistically the chances
of another disk failure or a URE (UnRecoverable Error) are quite high.


esarfl at gmail

Jul 27, 2012, 12:01 PM

Post #3 of 25 (1546 views)
Permalink
Re: Recording directly to RAID [In reply to]
On Fri, Jul 27, 2012 at 12:27 PM, David Engel <david [at] istwok> wrote:
> Is anyone recording directly to RAID? If so, what type/size, how many
> simultaneous recordings and how well does it work?

I started with a 3-desk RAID5 full of 2TB disks. This setup seemed to
choke around 3 recordings, 2 commercial flagging, and 1 playback (it
would probably stumble without the playback, too).

I had a spare 1.5TB disk, so I added it to the storage group. I
haven't had any IO issues since.

I now have three storage groups:

Default:
/raid/recordings
/mnt/wdgreen15/recordings

Important:
/raid/recordings

LowPriority:
/mnt/wdgreen15/recordings

Almost everything I record is set to Default, but occasionally I will
set something to Important if I want it to survive a drive crash.
Sometimes I will move files from /mnt/wdgreen15/recordings to
/raid/recordings if I decide they are important to me. When Myth fails
to find the file at the specified path, it tries the other storage
group path(s) and succeeds. This way I have some redundancy for
important recordings. I would not care if I lost the independent 1.5TB
disk (or even the whole array)--it's just TV.


Tom
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imntreal at gmail

Jul 27, 2012, 12:08 PM

Post #4 of 25 (1544 views)
Permalink
Re: Recording directly to RAID [In reply to]
On Fri, Jul 27, 2012 at 2:27 PM, David Engel <david [at] istwok> wrote:
> Is anyone recording directly to RAID? If so, what type/size, how many
> simultaneous recordings and how well does it work?

I'm about to attempt to move both the OS, and disk where I do my
recording to a btrfs RAID 1 array of two SSDs. If that fails, I'll
probably try mdadm before hardware RAID, as I want this to be portable
if I swap the disk controller. Either way, I wanted mirroring for my
OS drive. I have a cron job setup that moves all recordings not in
use to a directory in a backup storage group on a 6 TB RAID 10 btrfs
array. I can't wait until RAID 5/6 for btrfs is finalized. What a
waste of space.

=-Jameson
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imntreal at gmail

Jul 27, 2012, 12:09 PM

Post #5 of 25 (1548 views)
Permalink
Re: Recording directly to RAID [In reply to]
On Fri, Jul 27, 2012 at 3:08 PM, Jameson <imntreal [at] gmail> wrote:
> On Fri, Jul 27, 2012 at 2:27 PM, David Engel <david [at] istwok> wrote:
>> Is anyone recording directly to RAID? If so, what type/size, how many
>> simultaneous recordings and how well does it work?
>
> I'm about to attempt to move both the OS, and disk where I do my
> recording to a btrfs RAID 1 array of two SSDs. If that fails, I'll
> probably try mdadm before hardware RAID, as I want this to be portable
> if I swap the disk controller. Either way, I wanted mirroring for my
> OS drive. I have a cron job setup that moves all recordings not in
> use to a directory in a backup storage group on a 6 TB RAID 10 btrfs
> array. I can't wait until RAID 5/6 for btrfs is finalized. What a
> waste of space.

I forgot to mention that I haven't figured out why, but recording
directly to my RAID 10 btrfs array chokes with only one recording.
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mythtv at tapanitarvainen

Jul 27, 2012, 1:15 PM

Post #6 of 25 (1536 views)
Permalink
Re: Recording directly to RAID [In reply to]
On Jul 27 13:27, David Engel (david [at] istwok) wrote:

> Is anyone recording directly to RAID? If so, what type/size, how many
> simultaneous recordings and how well does it work?

I've got a 5-disk RAID10 array in a fileserver used by
mythtv over NFS in addition to one local disk.
It can handle 10 simultaneous recordings without blinking.
(Or at least could once when I tested it - besides that I don't
think I've ever had more than five.)

And the disks aren't particularly fast either: 2TB Seagate Green
5900 rpm disks. But in the RAID10 setup they can pull rather
impressive speeds, writing over 300MB/s and reading 600MB/s
for single stream, and what's more, multiple simultaneous
processes get almost the same in aggregate.
Key there was using offset (o2) layout with biggish (4M) block size -
defaults are rather ridiculous.

Enough RAM and good SATA controller are also important.
(I like LSI HBA cards.)

> The two configurations I'm primarily considering are either a 4-disk
> RAID6 array or a pair of 2-disk RAID1 mirrors. While the RAID6
> configuration should be more reliable, in theory, I'm leaning toward
> the RAID1 configuration. One big reason, and the reason for this
> email, is the increased seek load on all drives with RAID6 when 4 or
> more recordings are in-progress.

With four disks I'd go for RAID10 without question.

--
Tapani Tarvainen
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adeffs.mythtv at gmail

Jul 27, 2012, 1:27 PM

Post #7 of 25 (1536 views)
Permalink
Re: Recording directly to RAID [In reply to]
On Fri, Jul 27, 2012 at 4:15 PM, Tapani Tarvainen
<mythtv [at] tapanitarvainen> wrote:
> On Jul 27 13:27, David Engel (david [at] istwok) wrote:
>> Is anyone recording directly to RAID? If so, what type/size, how many
>> simultaneous recordings and how well does it work?
>
> I've got a 5-disk RAID10 array in a fileserver used by
> mythtv over NFS in addition to one local disk.
> It can handle 10 simultaneous recordings without blinking.
> (Or at least could once when I tested it - besides that I don't
> think I've ever had more than five.)
>
> And the disks aren't particularly fast either: 2TB Seagate Green
> 5900 rpm disks. But in the RAID10 setup they can pull rather
> impressive speeds, writing over 300MB/s and reading 600MB/s
> for single stream, and what's more, multiple simultaneous
> processes get almost the same in aggregate.
> Key there was using offset (o2) layout with biggish (4M) block size -
> defaults are rather ridiculous.
>
> Enough RAM and good SATA controller are also important.
> (I like LSI HBA cards.)
>
>> The two configurations I'm primarily considering are either a 4-disk
>> RAID6 array or a pair of 2-disk RAID1 mirrors. While the RAID6
>> configuration should be more reliable, in theory, I'm leaning toward
>> the RAID1 configuration. One big reason, and the reason for this
>> email, is the increased seek load on all drives with RAID6 when 4 or
>> more recordings are in-progress.
>
> With four disks I'd go for RAID10 without question.

I used to have a 6 disc RAID5 for recordings. I left that to go with
storage groups instead. I actually increased my overall read write
speeds with multiple recordings/playbacks. At peak times I can have
7-8 recordings with up to 3 playbacks and 4 commflag sessions going on
at once "without skipping a beat".

RAID, unless redundancy is required of your recordings, is not
necessary since the introduction of storage groups.

--
Steve
http://www.mythtv.org/wiki/User:Steveadeff
Before you ask, read the FAQ!
http://www.mythtv.org/wiki/Frequently_Asked_Questions
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joe at thefrys

Jul 27, 2012, 3:58 PM

Post #8 of 25 (1531 views)
Permalink
Re: Recording directly to RAID [In reply to]
> Is anyone recording directly to RAID? If so, what type/size, how many
> simultaneous recordings and how well does it work?
>
> I'm currently using 3 independent drives for recordings. The primary
> reason to use independent drives has been to spread in-progress
> recordings across multiple spindles. It's fairly common for me to
> have 3 or 4 simultaneous recordings and 6 is not unheard of during
> some busy parts ot the year.
>
> While my drives currently aren't on death's door, they are a few years
> old now and I have had a handful of scares in the last few monoths
> where various drives (or controllers) have gone wonky. Fonrtunately,
> the wonkyness has only been temporary and I haven't lost anything of
> real importance. Still I'm spooked enough to seriously consider
> adding some redundancy.
>
> The two configurations I'm primarily considering are either a 4-disk
> RAID6 array or a pair of 2-disk RAID1 mirrors. While the RAID6
> configuration should be more reliable, in theory, I'm leaning toward
> the RAID1 configuration. One big reason, and the reason for this
> email, is the increased seek load on all drives with RAID6 when 4 or
> more recordings are in-progress.
>

First of all... with 4 drives, a RAID 10 or 01 would be best. But do
yourself a favor and DONT USE RAID.

I ran a variety of RAID configurations over the years and have finally seen
the error of my ways.

Mythtv works best with multiple drives in it's storage groups and does a
wonderful job spreading the load out. With my arrays, I would hear the
drives thrashing as I recorded 2-3 channels, watched one recording, and had
a commflag running, the heads were all over the disks; because the array
used all of the disks for each of 5 processes. Now, with
4 independent disks, typically each disk is handling one or 2 of those
processes... which rarely taxes the drive.

Additionally, software raid has significant overhead... so a few percent of
my processor was dedicated to just handling the IO.

Finally, RAID does nothing for you if your data is corrupted. For example
I ran a transcode job on an entire season of a show and realized afterward
that the transcoded shows were unwatchable. RAID can't help you get them
back... really killed the WAF.

So I broke my arrays and now only back up the stuff I actually need (use
storage groups to keep important and unimportant recordings organized) to
an external drive using rsnapshot (highly recommended). Additionally, I
copy my database and some other important data off to an internet based
service.

If you have 4 drives and are willing to lose 50% for redundancy... a real
backup is a far better use of that extra space than RAID will ever be.

I bought a 2TB external esata drive just for this purpose so I could use my
existing drives for recordings.... so I doubled my storage with that
purchase and improved the performance and reliability of my system at the
same time.

Recovery is easy... if I lose a drive, I can simply copy the recordings
from the backup into any of the existing storage groups, assuming there is
room... or add the backup folder to the storage group until a new drive
arrives.


raymond at wagnerrp

Jul 27, 2012, 4:14 PM

Post #9 of 25 (1535 views)
Permalink
Re: Recording directly to RAID [In reply to]
On 7/27/2012 16:15, Tapani Tarvainen wrote:
> I've got a 5-disk RAID10 array

Huh? Are you counting a hot spare as part of the array?
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larry.sanderson at gmail

Jul 27, 2012, 4:32 PM

Post #10 of 25 (1526 views)
Permalink
Re: Recording directly to RAID [In reply to]
On Fri, Jul 27, 2012 at 4:14 PM, Raymond Wagner <raymond [at] wagnerrp>wrote:

> On 7/27/2012 16:15, Tapani Tarvainen wrote:
>
>> I've got a 5-disk RAID10 array
>>
>
> Huh? Are you counting a hot spare as part of the array?


I believe this is possible with the linux raid10 driver... rather than just
being a raid-0 of raid-1 devices, it instead cycles through the devices
putting down first a block, then it's mirror. In this way, it can use an
odd # of devices (including just 3). Check it out:
http://en.wikipedia.org/wiki/Non-standard_RAID_levels#Linux_MD_RAID_10

with five devices, it would look like:

D1 D2 D3 D4 D5
--------------
A1 A1 A2 A2 A3
A3 A4 A4 A5 A5
...


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raymond at wagnerrp

Jul 27, 2012, 6:49 PM

Post #11 of 25 (1521 views)
Permalink
Re: Recording directly to RAID [In reply to]
On 7/27/2012 19:32, Larry Sanderson wrote:
> On Fri, Jul 27, 2012 at 4:14 PM, Raymond Wagner <raymond [at] wagnerrp
> <mailto:raymond [at] wagnerrp>> wrote:
>
>> On 7/27/2012 16:15, Tapani Tarvainen wrote:
>>
>>> I've got a 5-disk RAID10 array
>>
>> Huh? Are you counting a hot spare as part of the array?
>
>
> I believe this is possible with the linux raid10 driver... rather than
> just being a raid-0 of raid-1 devices, it instead cycles through the
> devices putting down first a block, then it's mirror. In this way, it
> can use an odd # of devices (including just 3).

I've always heard that referred to as RAID 1E.
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mythtv at tapanitarvainen

Jul 27, 2012, 9:43 PM

Post #12 of 25 (1519 views)
Permalink
Re: Recording directly to RAID [In reply to]
On Fri, Jul 27, 2012 at 07:14:57PM -0400, Raymond Wagner (raymond [at] wagnerrp) wrote:

> On 7/27/2012 16:15, Tapani Tarvainen wrote:
> >I've got a 5-disk RAID10 array
>
> Huh? Are you counting a hot spare as part of the array?

No. Linux RAID10 supports odd number of disks.

--
Tapani Tarvainen
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mythtv at tapanitarvainen

Jul 27, 2012, 10:11 PM

Post #13 of 25 (1518 views)
Permalink
Re: Recording directly to RAID [In reply to]
On Fri, Jul 27, 2012 at 06:58:40PM -0400, Joseph Fry (joe [at] thefrys) wrote:

> First of all... with 4 drives, a RAID 10 or 01 would be best.

Yes. (With Linux mdadm, RAID10, period. With hardware RAID, it depends.)
In particular, RAID5 and RAID6 are poor choices for any situation where
write speed is critical (and even RAID10 needs non-default parameters
to do well).

> But do yourself a favor and DONT USE RAID.
>
> I ran a variety of RAID configurations over the years and have finally seen
> the error of my ways.

> Mythtv works best with multiple drives in it's storage groups and does a
> wonderful job spreading the load out.

True: MythTV does work quite well with independent disks, and for
MythTV-only use RAID may not pay off. But it's not quite that
clear-cut as you suggest.

> With my arrays, I would hear the drives thrashing as I recorded 2-3
> channels, watched one recording, and had a commflag running, the
> heads were all over the disks; because the array used all of the
> disks for each of 5 processes.

That can happen - getting a RAID array to be efficient with
multiple processes takes some tuning, you need more RAM
for buffering and good SATA card(s).

But my 5-disk RAID10 server can handle everything I can throw
at it over gigabit network, including several mythtv recordings
at the same time with lots of other stuff going on.
There's no way I could've gotten similar performance without RAID.

If it was dedicated to MythTV use, it might not be worthwhile,
especially if you're not managing RAID arrays otherwise,
as it does add complexity to the system and getting it to
run well takes time and effort.

> Now, with 4 independent disks, typically each disk is handling one
> or 2 of those processes... which rarely taxes the drive.

Yes. For MythTV that should work well.
It is also very simple setup.

> Additionally, software raid has significant overhead... so a few percent of
> my processor was dedicated to just handling the IO.

That is hardly significant unless you're using Atom or similar
ultra-low-power CPU.

> Finally, RAID does nothing for you if your data is corrupted.

> If you have 4 drives and are willing to lose 50% for redundancy... a real
> backup is a far better use of that extra space than RAID will ever be.

Yes. RAID is no substitute for backups. But the opposite is also true.

What RAID gives you that no backup can is uptime: recording won't
stop because one disk dies. And that is really the only good reason
for considering RAID in a dedicated mythtv box.

RAID can also give serious performance boost, but as noted that takes some
tuning and in a mythtv-only setup set of single disks will do quite
well without any effort.

--
Tapani Tarvainen
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raymond at wagnerrp

Jul 27, 2012, 10:45 PM

Post #14 of 25 (1514 views)
Permalink
Re: Recording directly to RAID [In reply to]
On 7/28/2012 01:11, Tapani Tarvainen wrote:
> That can happen - getting a RAID array to be efficient with
> multiple processes takes some tuning, you need more RAM
> for buffering and good SATA card(s).

MythTV has a 1 second sync loop for each file writer, specifically to
prevent complications that can occur from significant buffering. Adding
additional RAM won't help much.
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joe at thefrys

Jul 28, 2012, 2:01 PM

Post #15 of 25 (1498 views)
Permalink
Re: Recording directly to RAID [In reply to]
>
>
> > But do yourself a favor and DONT USE RAID.
> >
> > I ran a variety of RAID configurations over the years and have finally
> seen
> > the error of my ways.
>
> > Mythtv works best with multiple drives in it's storage groups and does a
> > wonderful job spreading the load out.
>
> True: MythTV does work quite well with independent disks, and for
> MythTV-only use RAID may not pay off. But it's not quite that
> clear-cut as you suggest.
>
> > With my arrays, I would hear the drives thrashing as I recorded 2-3
> > channels, watched one recording, and had a commflag running, the
> > heads were all over the disks; because the array used all of the
> > disks for each of 5 processes.
>
> That can happen - getting a RAID array to be efficient with
> multiple processes takes some tuning, you need more RAM
> for buffering and good SATA card(s).
>
> But my 5-disk RAID10 server can handle everything I can throw
> at it over gigabit network, including several mythtv recordings
> at the same time with lots of other stuff going on.
> There's no way I could've gotten similar performance without RAID.
>
> If it was dedicated to MythTV use, it might not be worthwhile,
> especially if you're not managing RAID arrays otherwise,
> as it does add complexity to the system and getting it to
> run well takes time and effort.
>

I am speaking in context of a Myth backend... if they have additional needs
that warrant the increased bandwidth for a single read/write operation that
is a different story altogether. A typical disk can support multiple
recording and playback operations and the data rates we see in modern
media, so bandwidth saturation isn't an issue.

Additionally, most RAID arrays bandwidth is lower than the combined JBOD
bandwidth (perhaps not with a high end controller or ideal workload). But
doing X reads/writes across X individual disks will likely be faster than
doing X reads/writes to a single array (if X is greater than 2 or 3).


> Additionally, software raid has significant overhead... so a few percent
> of
> > my processor was dedicated to just handling the IO.
>
> That is hardly significant unless you're using Atom or similar
> ultra-low-power CPU.


Depends upon how you define significant. I noticed my commflagging and
transcoding jobs complete between 5 & 10% faster after splitting the array.
I know it is only a couple % of processor time... but I don't believe that
it's all about processor time. My experience makes me believe that some
processes may only use 1 or 2% cpu time but have a substantially greater
impact on a CPU limited process; software raid is one of those. I suspect
it has to do with cache or RAM bandwidth/utilization, though I would think
that DMA would limit that impact.


> What RAID gives you that no backup can is uptime: recording won't
> stop because one disk dies. And that is really the only good reason
> for considering RAID in a dedicated mythtv box.
>

Very few, mythtv users need a HA (high availability) myth server. While it
stinks to miss your favorite show, or even a couple of them... the cost of
a RAID array (even software RAID) far exceeds the loss. Perhaps if your
career or business depended upon seeing a particular recording... but then
there are so many other things that you would need to put in place to
ensure that you didn't miss it (redundant tuners, cable/antenna feeds, that
RAID would only be the start).


>
> RAID can also give serious performance boost, but as noted that takes some
> tuning and in a mythtv-only setup set of single disks will do quite
> well without any effort.
>

It is false that RAID gives you a performance boost. No RAID array can
sustain transfers faster than the transfer rate of the individual drives
combined. Try it some time; create the fastest array you can and stream
100GB of zeros to it. Now split the array and write 100GB/# of drives to
each drive simultaneously. Unless your controller simply can't handle
standalone disks well, you should find that the same amount of data can be
written to your drives faster than it could when they are RAIDed. All RAID
has a performance HIT not a boost.

Where RAID is faster is doing single operations. Rarely, such as when
concurrent recordings seem to all be going to the same disk (because of the
logic behind the storage group load balancing) a RAID array would have the
advantage... but MOST of the time, the load is spread pretty equally, so I
am getting better performance than what RAID can offer.


jyavenard at gmail

Jul 28, 2012, 7:43 PM

Post #16 of 25 (1494 views)
Permalink
Re: Recording directly to RAID [In reply to]
Hi

On 28 July 2012 15:11, Tapani Tarvainen <mythtv [at] tapanitarvainen> wrote:
> Yes. (With Linux mdadm, RAID10, period. With hardware RAID, it depends.)
> In particular, RAID5 and RAID6 are poor choices for any situation where
> write speed is critical (and even RAID10 needs non-default parameters
> to do well).

I always find it particularly amusing reading about the use of extremes like...

I doubt Myth will ever require "critical" write speed.

So say you're recording 10 streams at the maximum bitrates of
22Mbit/s, 220Mbit/s is a figure that will unlikely ever see, as a
typical DVB stream is more around 15Mbit/s for some HD channels, and
that's if you're lucky....

RAID5 is perfectly suitable for home use, no point disregarding with
statement like RAID10 "period".

The most I've ever recorded at once was 6 streams, most of them SD so
6-7Mbit/s, my RAID5 array with green drives had no issue handling it,
recordings were just fine.

RAID10 is a waste of disk space IMHO if all you're after is disk
redundancy and big storage capacity.

We're not talking about enterprise-class SQL server here...

What next? so only RAID10 SSD disks should be used otherwise
everything is too slow "period" ?
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myrdhn at gmail

Jul 28, 2012, 8:07 PM

Post #17 of 25 (1490 views)
Permalink
Re: Recording directly to RAID [In reply to]
-----Original Message-----
From: mythtv-users-bounces [at] mythtv
[mailto:mythtv-users-bounces [at] mythtv] On Behalf Of Jean-Yves Avenard
Sent: Saturday, July 28, 2012 10:44 PM
To: Discussion about MythTV
Subject: Re: [mythtv-users] Recording directly to RAID

Hi

On 28 July 2012 15:11, Tapani Tarvainen <mythtv [at] tapanitarvainen> wrote:
> Yes. (With Linux mdadm, RAID10, period. With hardware RAID, it
> depends.) In particular, RAID5 and RAID6 are poor choices for any
> situation where write speed is critical (and even RAID10 needs
> non-default parameters to do well).

I always find it particularly amusing reading about the use of extremes
like...

I doubt Myth will ever require "critical" write speed.

So say you're recording 10 streams at the maximum bitrates of 22Mbit/s,
220Mbit/s is a figure that will unlikely ever see, as a typical DVB stream
is more around 15Mbit/s for some HD channels, and that's if you're lucky....

RAID5 is perfectly suitable for home use, no point disregarding with
statement like RAID10 "period".

The most I've ever recorded at once was 6 streams, most of them SD so
6-7Mbit/s, my RAID5 array with green drives had no issue handling it,
recordings were just fine.

RAID10 is a waste of disk space IMHO if all you're after is disk redundancy
and big storage capacity.

We're not talking about enterprise-class SQL server here...

What next? so only RAID10 SSD disks should be used otherwise everything is
too slow "period" ?
_______________________________________________

LOL. I always love when this gets brought up.
My server has 2 mirrored OS drives (mdadm), and 4 drives in RAID5 (LSI
Controller w/ battery) for media storage.
I have 7 potential HD capture sources, 3 of which could be multi captures
due to multiplex sharing. Never have I come close to maxing out the write
performance on the drives. I actually just decided to shutdown 3 of my
sources because I haven't been coming close to needing them all.

MarcT

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mythtv at tapanitarvainen

Jul 28, 2012, 9:53 PM

Post #18 of 25 (1493 views)
Permalink
Re: Recording directly to RAID [In reply to]
On Sat, Jul 28, 2012 at 05:01:56PM -0400, Joseph Fry (joe [at] thefrys) wrote:

> I am speaking in context of a Myth backend... if they have additional needs
> that warrant the increased bandwidth for a single read/write operation that
> is a different story altogether.

Yes. My apologies for rambling about more general RAID issues.

> A typical disk can support multiple recording and playback
> operations and the data rates we see in modern media, so bandwidth
> saturation isn't an issue.

Probably not. Which is why I said:

> > What RAID gives you that no backup can is uptime: recording won't
> > stop because one disk dies. And that is really the only good reason
> > for considering RAID in a dedicated mythtv box.

And for that, the best option for four disks would probably
be two independent RAID1 packs, as originally suggested.

(RAID6 might slow it down so much it would begin to matter.)

> Very few, mythtv users need a HA (high availability) myth server.

I wouldn't call a single box with RAID disks a HA server...

Anyway, in my experience disks are the part that breaks most often
in computers, and I like the ease of replacing them in RAID.
But I admit wouldn't bother with mythtv - indeed I haven't,
as noted, I have a single disk in my mythtv server, I just balance
the load by using the file server as a 2nd disk.
From the mythtv server's perspective that's about the same
as two individual local disks, and fast enough.

> While it stinks to miss your favorite show, or even a couple of
> them... the cost of a RAID array (even software RAID) far exceeds
> the loss.

With software RAID its just the cost of disks and the time it takes to
set it up, and if you count your time for real, you wouldn't be using
mythtv in the first place. :-)

And disks are still fairly cheap. Whether they're worth it,
well, depends, I guess.

> > RAID can also give serious performance boost

> It is false that RAID gives you a performance boost.

In the specific context of mythtv, agreed.

> No RAID array can sustain transfers faster than the transfer rate of
> the individual drives combined.

That is obvious, but it doesn't mean RAID can't boost performance.

> Where RAID is faster is doing single operations.

In general, it can be faster whenever the number of disks exceeds the
number of simultaneous operations. Note: _can_ be, it may well be
slower as well, sometimes by design - e.g., RAID5 and RAID6 are
notoriously slow in writing.

> Rarely, such as when concurrent recordings seem to all be going to
> the same disk (because of the logic behind the storage group load
> balancing) a RAID array would have the advantage... but MOST of the
> time, the load is spread pretty equally, so I am getting better
> performance than what RAID can offer.

Yes, given that disks are so fast that a single disk is always fast
enough for one recording, by a huge margin.
And while, say, a 4-disk RAID10 array would be faster for two parallel
recordings than four independent disks (two of them would be idle),
it does not seem likely that there'll be any super-HD format in the near
future where that'd matter. :-)

--
Tapani Tarvainen
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mythtv at tapanitarvainen

Jul 28, 2012, 9:59 PM

Post #19 of 25 (1489 views)
Permalink
Re: Recording directly to RAID [In reply to]
On Sun, Jul 29, 2012 at 12:43:32PM +1000, Jean-Yves Avenard (jyavenard [at] gmail) wrote:

> RAID5 is perfectly suitable for home use, no point disregarding with
> statement like RAID10 "period".

> RAID10 is a waste of disk space IMHO if all you're after is disk
> redundancy and big storage capacity.

While RAID5 is fast enough for many uses, what I particularly dislike
in it is the horribly slow rebuild time and loss of performance during it.
I have seen RAID5 arrays dies because another disk died during rebuild.

> What next? so only RAID10 SSD disks should be used otherwise
> everything is too slow "period" ?

There's no point in spending on more speed than you need, of course.
Ditto reliability. It's up to you to determine what you need and
how much you want to pay for it.

--
Tapani Tarvainen
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raymond at wagnerrp

Jul 28, 2012, 11:17 PM

Post #20 of 25 (1481 views)
Permalink
Re: Recording directly to RAID [In reply to]
On 7/28/2012 23:07, Marc Tousignant wrote:
> My server has 2 mirrored OS drives (mdadm), and 4 drives in RAID5 (LSI
> Controller w/ battery) for media storage.
> I have 7 potential HD capture sources, 3 of which could be multi captures
> due to multiplex sharing. Never have I come close to maxing out the write
> performance on the drives.

Now you're really not playing fair here. The whole issue is one of seek
latency, with multiple writes per second per recording, multiple
recordings, and amplification due to freespace fragmentation. When you
bring your battery-backed RAID controller into the equation, you're
instantly dropping those seek latencies by 5-6 orders of magnitude, and
giving yourself a good minute or two of buffer to linearize those writes.
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mythtv at tapanitarvainen

Jul 29, 2012, 12:20 AM

Post #21 of 25 (1483 views)
Permalink
Re: Recording directly to RAID [In reply to]
> While RAID5 is fast enough for many uses, what I particularly dislike
> in it is the horribly slow rebuild time and loss of performance during it.
> I have seen RAID5 arrays dies because another disk died during rebuild.

One more point here: in particular with big, multi-TB disks RAID5 is
of dubious value. The heavy disk activity dyring the long rebuild is
often the trigger that kills the second disk.
In MythTV context, independent disks might actually give better
reliability: if you have RAID5 die because of two disks failing you
lose everything in the array, with independent disks you lose only
what's on those disks.

--
Tapani Tarvainen
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joe at thefrys

Jul 29, 2012, 2:49 AM

Post #22 of 25 (1478 views)
Permalink
Re: Recording directly to RAID [In reply to]
> > While RAID5 is fast enough for many uses, what I particularly dislike
> > in it is the horribly slow rebuild time and loss of performance during
> it.
> > I have seen RAID5 arrays dies because another disk died during rebuild.
>
> One more point here: in particular with big, multi-TB disks RAID5 is
> of dubious value. The heavy disk activity dyring the long rebuild is
> often the trigger that kills the second disk.
> In MythTV context, independent disks might actually give better
> reliability: if you have RAID5 die because of two disks failing you
> lose everything in the array, with independent disks you lose only
> what's on those disks.


My apologies for starting yet another pro/con RAID discussion.

I will readily admit that I was quite fond of my RAID solution when I had
it... thought I was being smart using it, and even advocated it to others.
Now that I have "seen the light" I realize that for various reasons I was
getting no real advantage running RAID, and several potential disadvantages.

I do still mirror my root partition, simply so I don't need to rebuild my
server if a drive fails. I'll still need to restore any lost data, but at
least I don't need to rebuild and configure the server.

Ultimately, when the choice is between dedicating a drive for parity on an
array, or using that same drive to backup important media (rips of dvd's
that have been scratched, wedding videos, news stories about my kids,
etc.)... choose the backup.

If you really want to do RAID, go for it, MythTV works quite well on an
array. However I felt that it made things like adding/replacing drives,
recovering from a power failure, etc. far more frustrating.


jedi at mishnet

Jul 29, 2012, 1:21 PM

Post #23 of 25 (1451 views)
Permalink
Re: Recording directly to RAID [In reply to]
On Sun, Jul 29, 2012 at 07:53:58AM +0300, Tapani Tarvainen wrote:
> On Sat, Jul 28, 2012 at 05:01:56PM -0400, Joseph Fry (joe [at] thefrys) wrote:
>
> > I am speaking in context of a Myth backend... if they have additional needs
> > that warrant the increased bandwidth for a single read/write operation that
> > is a different story altogether.
>
> Yes. My apologies for rambling about more general RAID issues.
>
> > A typical disk can support multiple recording and playback
> > operations and the data rates we see in modern media, so bandwidth
> > saturation isn't an issue.
>
> Probably not. Which is why I said:
>
> > > What RAID gives you that no backup can is uptime: recording won't
> > > stop because one disk dies. And that is really the only good reason
> > > for considering RAID in a dedicated mythtv box.
>
> And for that, the best option for four disks would probably
> be two independent RAID1 packs, as originally suggested.
>
> (RAID6 might slow it down so much it would begin to matter.)
>
> > Very few, mythtv users need a HA (high availability) myth server.
>
> I wouldn't call a single box with RAID disks a HA server...
>
> Anyway, in my experience disks are the part that breaks most often
> in computers, and I like the ease of replacing them in RAID.

Just use a hot swap tray. That's what I do.

Chances are you don't care about your recordings so much that it
would be such a great tragedy if they all 'went poof' one day. So you
throw in another disk and just go on about your merry way.

[deletia]

Needless to say, I have zero redundancy on my recordings.

As far as performance goes, my performance bottleneck is the network since
nearly everything on my setup is happening across the network.
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blammo.doh at gmail

Jul 29, 2012, 10:16 PM

Post #24 of 25 (1446 views)
Permalink
Re: Recording directly to RAID [In reply to]
Since this thread seems to have wandered off a bit, I'll reply to the
original poster.

On Fri, Jul 27, 2012 at 11:27 AM, David Engel <david [at] istwok> wrote:
> Is anyone recording directly to RAID? If so, what type/size, how many
> simultaneous recordings and how well does it work?

Yes, I made the jump to HWRaid5 a long time ago on my Mythbackend, and
haven't looked back.

I originally played with SDraid, but left that behind due to IO issues
and stability. I started with HDRaid, with a 3ware 9500S-12 with 6x160
SATA drives. With proper tuning (elevator=deadline, xfs stripe/stride)
I was able to do 6 SD recordings plus realtime commercial flagging and
never drop/glitch recordings. Moved to 12x160gig drives, then later
12x500GB drives, then to 12x1GB drives, 2 HDHomerun, all with no
issues.

Next jump was to 12x2TB seagate green drives, and an ARECA card (I
wanted RAID6, and the out-of-band RAID management via the ethernet
port on the card). I would have shifted to ESXi, had it been able to
handle >2TB volumes. Instead I moved to ProxMox using KVM. I
virtualized the old backend, and exported the raid array via NFS to
the VM. Knock on wood, but I haven't lost a single 2TB drive in 2+
years.

I'm now up to 8 tuners, and I with all 8 going, realtime commercial
flagging, two frontends, and misc samba use, the raid array purrs
along with no issues, little-no io-wait. (The olympics are giving it a
good workout). I recently converted mythbackend to openVZ container,
and it's running better than ever. (To be fair, the machine is 2 x
quad-core opteron, with 32GB of ram, on a tyan server motherboard..
but CnQ works great, and the power-footprint is about 150W mostly
idle, which is amazing considering the drives/ram/etc)

So short version, yes, Raid is usable, possible, and worth the time,
cost, and effort in my opinion.
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blammo.doh at gmail

Jul 30, 2012, 4:27 PM

Post #25 of 25 (1428 views)
Permalink
Re: Recording directly to RAID [In reply to]
I know you're not supposed to reply to yourself, but in this case I'm
making fun of myself...

On Sun, Jul 29, 2012 at 10:16 PM, Blammo <blammo.doh [at] gmail> wrote:
> I originally played with SDraid, but left that behind due to IO issues
> and stability. I started with HDRaid, with a 3ware 9500S-12 with 6x160

Obviously I meant Software Raid (SWRaid) and Hardware Raid (HWRaid)
not SD/HD... brain wasn't fully engaged yet.
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