speedtoys.racing at gmail
Mar 19, 2012, 10:45 PM
Post #2 of 6
Yup, its the best you can do at this time.
Re: Physical reallocate for a deduped volume
[In reply to]
DO NOT do a reallocate -A.
On Mon, Mar 19, 2012 at 10:12 PM, Mathew Kilham <
matt.kilham [at] strattonfinance> wrote:
> Hi Toasters,
> I've posed the following question to NetApp support but wasn't too
> confident in the answer I received, so I was hoping the knowledgeable folks
> on this list could assist? Also, before I begin, please excuse any
> ignorance or NetApp faux pas on my behalf, I'm far from an expert at
> administering NetApp filers.
> We have a FAS2050C running DOT 7.3.4. We recently added an external disk
> shelf to the filer and performed a bit of reorganisation, which has left us
> with two unused disks in the internal shelf. We want to add these two disks
> to an existing 16-disk aggregate by expanding it's RAID group size from 16
> to 18. The aggregate is currently ~80% full and contains 5 flexvols, most
> of which have de-dupe and snapshots enabled.
> I understand that after expanding the RG / aggregate we need to run a
> physical reallocate or else we will end up with a significant "hot spot" on
> the newly-added disks. I also understand that we need ensure DOT only does
> a physical redistribution of the blocks across all disks in the aggregate
> and does not try to otherwise rearrange them, otherwise we risk
> "un-deduping" our deduplicated data and/or greatly increasing the space
> consumed by our snapshots.
> First question: I believe the command we need to run to perform the above
> style of reallocation is "reallocate start -f -p /vol/<volname>" - is that
> I noted, however, that there is a warning with regards to the "reallocate
> -p" command in the release notes of 7.3.x. Unfortunately I can't find a
> link anymore, but it reads:
> "A file reallocation scan using reallocate start or reallocate start -p
> does not rearrange blocks that are shared between files by deduplication on
> deduplicated volumes. Since a file reallocation scan does not predictably
> improve read performance when used on deduplicated volumes, performing file
> reallocation on deduplicated volumes is not recommended. Instead, for files
> to benefit from the reallocation scan, they should be stored on volumes
> that are not enabled for deduplication."
> I then queried with NetApp how we could perform a physical reallocate on
> the aggregate to avoid a hot spot, given the above statement that
> "reallocate start -p does not rearrange blocks that are shared between
> files by deduplication on deduplicated volumes", and almost all of our data
> is deduped.
> After a bit of to-ing and fro-ing with NetApp, the final suggestion was as
> "I’ve talked to two senior resources so far about this. The suggested
> steps are as follows>>
> #1: add your disks to the aggregate, and then grow the volume.
> #2: Turn off Deduplication, ( no, this will not revert to the 2.5 gigs
> from the one gig ratio it is now and suddenly cause you to be out of space)
> this will only stop deduplication from this point forward until it is
> re-enabled later. Everything that is currently deduped will stay the ratio
> it is.
> #3: Perform the reallocate –p command as specified in the KB’s sent
> #4: After all above steps are performed and reallocate is done, re-enable
> deduplication on said volumes."
> Second question: Does this sound kosher - will this actually result in a
> physical reallocate being performed, or will the deduped blocks still be
> skipped because they're already de-duplicated, regardless of whether dedupe
> is enabled for new data on the volume or not?
> Third question: If yes to the previous question, is there any risk that
> our deduped data get un-deduped by performing the suggested steps above?
> Fourth and final question: Is there any way to measure how data is spread
> across disks in a Raid Group / aggregate, so that I can check that we get
> our expected results from the reallocate?
> Thanks everyone, really appreciate any feedback / assistance.
> Toasters mailing list
> Toasters [at] teaparty
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