[OpenIndiana-discuss] ZFS with Dedupication for NFS server

Toomas Soome Toomas.Soome at mls.ee
Thu Apr 21 22:07:00 UTC 2011

the basic math behind the scenes is following (and not entirely determined):

1. DTT data is kept in metadata part of ARC; 
2. metadata default max is arc_c_max / 4. 

note that you can rise that limit.

3. arc max is RAM - 1GB. 

so, if you have 8GB of ram, your arc max is 7GB and max metadata is 1.75GB.  so, with server with 8GB of ram, your server will store MAX 1.75GB DTT in arc. the DTT entry is told to take 250B.

now the tricky part - those numbers are max values; but you also need some space to store "normal" metadata, not just DTT. also, you cant really distinguish DTT from other metadata - that will leave space for some guessing unfortunately.

for perfomance consideration, even if you have enough ram and l2arc, the arc warmup time is more critical, as currently the l2arc contents will be lost on reboot and arc contents as well obviously - thats the downside of having dedupe integrated into filesystem.

On 22.04.2011, at 0:48, Eric D. Mudama wrote:

> On Thu, Apr 21 at 14:12, James Kohout wrote:
>> All,
>> Been running opensolaris 134 with a 9T RaidZ2 array as a backup server
>> in a  production environment.   Whenever I tried to turn the ZFS
>> deduplication I always had crashes and other issues, which I most likely
>> attributed to the know ZFS dedup bugs in 134.  Once I rebuild the pool
>> without dedup, things have been running great for several months without
>> a glitch.  As a result, I am highly confident it was not a hardware issue.
>> So looking to upgrade to io148 to be able to enable deduplication.  So
>> does have any experience running a ZFS RaidZ2 pool with deduplication in
>> a production environment?  Is  ZFS deduplication in oi148 considered
>> stable/production ready?  I would hate to break a working setup chasing a
>> feature that is not ready.
>> Any feedback, experience would be appreciated.
> The summary of list postings over the last 6 months is that dedup
> requires way more RAM and/or L2ARC than most people budgeted in order
> to work as smoothly as a non-dedup installation, and that when under
> budgeted in RAM/L2ARC, the performance of scrubs and snapshot deletion
> is atrocious.
> I don't have the math handy on the memory requirements, maybe someone
> can post that part of the summary.
> -- 
> Eric D. Mudama
> edmudama at bounceswoosh.org
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