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Ticket #10991: RFE: make recording raid friendly

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noreply at mythtv

Aug 11, 2012, 8:31 AM

Post #1 of 3 (80 views)
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Ticket #10991: RFE: make recording raid friendly

#10991: RFE: make recording raid friendly
--------------------------------------+------------------------
Reporter: dlbulk-mythtv@… | Owner: danielk
Type: Bug Report - General | Status: new
Priority: minor | Milestone: unknown
Component: MythTV - Recording | Version: 0.25-fixes
Severity: medium | Keywords:
Ticket locked: 0 |
--------------------------------------+------------------------
When recordings are written raid devices, it is often reporting that write
is too slow and eventually the recording stops after truncating the file.

Benchmarks on the raid device show that the throughput is far higher than
mythtv reports.

According to iostat output, when mythtv is recording, there is also lots
of read activity on all of the raid disks. It is happening even if there
are no read requests on the md device itself. This implies that there is
partial stripe writes happening. In this case, raid has to do a read in
order to completes the partial stripe write.

Two factors can trigger partial stripe writes:
1. write offset is not stripe aligned
2. io size is not a multiple of a stripe

Although there is no direct control of raid layout at application layer,
it helps if mythtv performs aligned writes only and uses an io size that's
a multiple of a raid stripe. The alignment should also be a multiple of
the raid stripe.

Yes, an underperforming disk can slow down writes. But partial stripe
writes make it even slower :)

--
Ticket URL: <http://code.mythtv.org/trac/ticket/10991>
MythTV <http://code.mythtv.org/trac>
MythTV Media Center
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noreply at mythtv

Aug 11, 2012, 8:49 AM

Post #2 of 3 (78 views)
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Re: Ticket #10991: RFE: make recording raid friendly [In reply to]

#10991: RFE: make recording raid friendly
----------------------------------+----------------------------
Reporter: dlbulk-mythtv@… | Owner: danielk
Type: Bug Report - General | Status: new
Priority: minor | Milestone: unknown
Component: MythTV - Recording | Version: 0.25-fixes
Severity: medium | Resolution:
Keywords: | Ticket locked: 0
----------------------------------+----------------------------

Comment (by dlbulk-mythtv@…):

observed io sizes used by recording:

{{{
Aug 11 07:45:02 ThreadedFileWriter.cpp:499 write(57528) cnt 31 total
1745392 -- took a long time, 1067 ms
Aug 11 07:45:19 ThreadedFileWriter.cpp:499 write(57716) cnt 57 total
3284548 -- took a long time, 1171 ms
Aug 11 07:45:32 ThreadedFileWriter.cpp:499 write(57528) cnt 42 total
2403768 -- took a long time, 1260 ms
Aug 11 07:47:19 ThreadedFileWriter.cpp:499 write(57528) cnt 42 total
2408656 -- took a long time, 1011 ms
Aug 11 07:59:31 ThreadedFileWriter.cpp:499 write(57904) cnt 38 total
2171212 -- took a long time, 1151 ms
Aug 11 08:04:40 ThreadedFileWriter.cpp:499 write(57528) cnt 109 total
6281456 -- took a long time, 1442 ms
Aug 11 08:04:40 ThreadedFileWriter.cpp:499 write(57528) cnt 54 total
3070228 -- took a long time, 1457 ms
Aug 11 08:04:52 ThreadedFileWriter.cpp:499 write(57528) cnt 73 total
4154800 -- took a long time, 1321 ms
Aug 11 08:06:28 ThreadedFileWriter.cpp:499 write(57528) cnt 43 total
2434600 -- took a long time, 1118 ms
Aug 11 08:16:21 ThreadedFileWriter.cpp:499 write(57528) cnt 31 total
1792580 -- took a long time, 1110 ms
Aug 11 08:16:22 ThreadedFileWriter.cpp:499 write(57528) cnt 58 total
3299400 -- took a long time, 1382 ms
Aug 11 08:21:30 ThreadedFileWriter.cpp:499 write(57528) cnt 77 total
4410668 -- took a long time, 1877 ms
Aug 11 08:36:44 ThreadedFileWriter.cpp:499 write(57716) cnt 44 total
2534804 -- took a long time, 1297 ms

(gdb) p/x 57528
$1 = 0xe0b8
(gdb) p/x 57716
$2 = 0xe174
(gdb) p/x 57904
$3 = 0xe230

}}}

--
Ticket URL: <http://code.mythtv.org/trac/ticket/10991#comment:1>
MythTV <http://code.mythtv.org/trac>
MythTV Media Center
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noreply at mythtv

Aug 11, 2012, 9:25 AM

Post #3 of 3 (73 views)
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Re: Ticket #10991: RFE: make recording raid friendly [In reply to]

#10991: RFE: make recording raid friendly
---------------------------+-----------------------------------------------
Reporter: dlbulk- | Owner: danielk
mythtv@… | Status: closed
Type: Bug Report - | Milestone: unknown
General | Version: 0.25-fixes
Priority: minor | Resolution: Feature request without patch
Component: MythTV - | Ticket locked: 0
Recording |
Severity: medium |
Keywords: |
---------------------------+-----------------------------------------------
Changes (by wagnerrp):

* status: new => closed
* resolution: => Feature request without patch

--
Ticket URL: <http://code.mythtv.org/trac/ticket/10991#comment:2>
MythTV <http://code.mythtv.org/trac>
MythTV Media Center
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