ONTAP Discussions

Huge CIFS snapshots after dedupe

acistmedical
19,205 Views

Somehow our CIFS snapshots get huge after dedupe.

For example, if i dont run dedpue for a week, my snaps are few hundred mb each, i take them twice a day, noon and midnight.

However, when i run dedupe, it somehow increases the size of a last snap. i just run full dedupe (took 6 h), my last snap was 230mb befrore the dedupe, the same snap is now 70Gb, after the dedupe had run.

Also, if i schedule dedupe to run at 11 and then run snap at 12 the snap is way bigger than if there is no dedupe scheduled at all.

Whats going on??

The same things happens on both controllers.

BTW, im running full dedupe on another CIFS volume now and i can actually see the last snap growing as dedupe progresses. It was 124Mb before i started, then i checked it at 39% of dedupe, it was 21Gb, now at 45% of dedupe  its grew to 26Gb.

what the ???????

25 REPLIES 25

acistmedical
4,302 Views

You are correct, dedupe doesnt run at all because there is not enough changes.

srichard
4,303 Views

@acistmedical:

What version of ONTAP are you running?

7.2 has all the (partially temporary, partially persistent) Dedupe Metadata in the FlexVol. Thus any snapshot taken with 7.2(.6.1+) on a sis-enabled flexvol will *always* be significantly larger than performing the same operation with 7.3.

The output of the "sis stat -lv" and "sis status -l" command (advanced mode) might be helpful in determining the reason for the larger than expected snapshots.

Part of this might be that snapshots get/got created while a sis run. Part might be do to intermediate (hourly) snapshots, which contain locked-in referenced for now deduped blocks.

If you are on 7.2, plan for an upgrade to 7.3 in order to get the best possible savings between snapshots and sis...

Are you running a PAM card (16G) in that system?

acistmedical
4,303 Views

We are using 7.2.5.1 but will upgrade to 7.3.2 very soon.

There is no possibility that snaps are running during dedupe because there is small amont of data that changes and dedupe takes only few min to complete.

What is PAM card, how can i tell if i have one?

srichard
4,303 Views

You would know to have a PAM card installed, if you had one (7.3 is required for the 16GB version, and 7.3.2 for the 256/512 GB versions).

But running sis with 7.2.5.1 can have some unexpected consequences, such as increased snapshot space. This is, as mentioned, due to the fact that prior to 7.3, sis metadata was saved within the FlexVol and therefore becomes part of the snapshot.

Ie. if you you store, say, 400 MB of new CIFS data, then this will require the storage of ~100 000 new fingerprints (temporary data; locked in intermediate snapshots). Once SIS runs, these 100 000 fingerprints (hashes) will be sorted and inserted in the already existing fingerprint database (re-writing that DB), while duplicate fingerprints are checked if the block is really the same (and the reference to that duplicate data block will be resolved - deduped).

Thus, even if all but a few of these 100 000 new blocks could be deduped, at least the new, sorted fingerprint DB will be re-written to a large extent. (if all blocks were present in the DB, no new sorted DB would need to be written). Depending on the amount of data, this alone might be a few dozend or even hundreds of megabytes - which get locked in with the next snapshot...

ONTAP 7.3 has architectural changes to much of the SIS stuff, and is no longer prone to this issue. The general recommendation for production, performance and most space-efficient SIS is to deploy this only after 7.3 (7.3.2P4 currently).

However, the recommendation of staging SIS and snapshots in such a way that sis is finished *before* snapshots are taken (especually those with long retention time), should still be heeded.

Keep this thread updated, once the ONTAP upgrade is done! I'm interested in hearing your comments then!

acistmedical
4,303 Views

After the upgrade to 7.3.2 everything works great.

I have a dedupe set to tun at 11 and 23 and snap to run as 12 an 0 and all is fine.

Snaps are small, dedupe doesnt affect snap sizes anymore.

Thanks guys for fixing it in this release.

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