[Lustre-discuss] number of inodes in zfs MDT
Anjana Kar
kar at psc.edu
Wed Jun 18 03:11:33 PDT 2014
We just finished testing zfs 0.6.3 and lustre 2.5.2 with zfs MDT & OSTs, but
still ran into the problem of running out of inodes on the MDT. The number
started at about 7.9M, and grew to 8.9M, but not beyond that, the MDT
being on a mirrored zpool of 2 80GB SSD drives. The filesystem size was 97TB
with 8 13TB raidz2 OSTs on a shared MDS/OSS node, and a second OSS.
It took ~5000 seconds to run out of inodes in our empty file test. But,
during that time it averaged about 1650/sec which is the best we've seen.
I'm not sure why the inodes have been an issue, but we ran out of time to
pursue this further.
Instead we have moved to ldiskfs MDT and zfs OSTs, with the same lustre/zfs
versions, and have a lot more inodes available.
Filesystem Inodes IUsed IFree IUse% Mounted on
x.x.x.x at o2ib:/iconfs
39049920 7455386 31594534 20% /iconfs
Performance has been reportedly better, but one problem was that when
the OSS nodes went down before the OSTs could be taken offline (as would
happen during a power outage), OSTs failed to mount after the reboot.
To get around that we added a zpool import -f line after the message
"Unexpected return code from import of pool $pool" in the lustre startup
script
so the pools mount, then ran the lustre startup script to start the
OSTs. If
there is a better way to handle this please let me know.
Another problem we ran into is that our 1.8.9 clients could not write
into the new
filesystem with lustre 2.5.60 which came from
git.hpdd.intel.com/fs/lustre-release.git.
Things worked after checking out "track -b b2_5 origin/b2_5", and
rebuilding kernel
for ldiskfs. OS on the lustre servers is CentOS 6.5, kernel 2.6.32-431.17.1.
Thanks again for all the responses.
-Anjana
On 06/12/2014 09:43 PM, Scott Nolin wrote:
> Just a note, I see zfs-0.6.3 has just been annoounced:
>
> https://groups.google.com/a/zfsonlinux.org/forum/#!topic/zfs-announce/Lj7xHtRVOM4
>
>
> I also see it is upgraded in the zfs/lustre repo.
>
> The changelog notes the default as changed to 3/4 arc_c_max and a
> variety of other fixes, many focusing on performance.
>
> So Anjana this is probably worth testing, especially if you're
> considering drastic measures.
>
> We upgraded for our MDS, so this file create issue is harder for us to
> test now (literally started testing writes this afternoon, and it's
> not degraded yet, so far at 20 million writes). Since your problem
> still happens fairly quickly I'm sure any information you have will be
> very helpful to add to LU-2476. And if it helps, it may save you some
> pain.
>
> We will likely install the upgrade but may not be able to test
> millions of writes any time soon, as the filesystem is needed for
> production.
>
> Regards,
> Scott
>
>
> On Thu, 12 Jun 2014 16:41:14 +0000
> "Dilger, Andreas" <andreas.dilger at intel.com> wrote:
>> It looks like you've already increased arc_meta_limit beyond the
>> default, which is c_max / 4. That was critical to performance in our
>> testing.
>>
>> There is also a patch from Brian that should help performance in your
>> case:
>> http://review.whamcloud.com/10237
>>
>> Cheers, Andreas
>>
>> On Jun 11, 2014, at 12:53, "Scott Nolin"
>> <scott.nolin at ssec.wisc.edu<mailto:scott.nolin at ssec.wisc.edu>> wrote:
>>
>> We tried a few arc tunables as noted here:
>>
>> https://jira.hpdd.intel.com/browse/LU-2476
>>
>> However, I didn't find any clear benefit in the long term. We were
>> just trying a few things without a lot of insight.
>>
>> Scott
>>
>> On 6/9/2014 12:37 PM, Anjana Kar wrote:
>> Thanks for all the input.
>>
>> Before we move away from zfs MDT, I was wondering if we can try
>> setting zfs
>> tunables to test the performance. Basically what's a value we can use
>> for
>> arc_meta_limit for our system? Are there are any others settings that
>> can
>> be changed?
>>
>> Generating small files on our current system, things started off at 500
>> files/sec,
>> then declined so it was about 1/20th of that after 2.45 million files.
>>
>> -Anjana
>>
>> On 06/09/2014 10:27 AM, Scott Nolin wrote:
>> We ran some scrub performance tests, and even without tunables set it
>> wasn't too bad, for our specific configuration. The main thing we did
>> was verify it made sense to scrub all OSTs simultaneously.
>>
>> Anyway, indeed scrub or resilver aren't about Defrag.
>>
>> Further, the mds performance issues aren't about fragmentation.
>>
>> A side note, it's probably ideal to stay below 80% due to
>> fragmentation for ldiskfs too or performance degrades.
>>
>> Sean, note I am dealing with specific issues for a very create intense
>> workload, and this is on the mds only where we may change. The data
>> integrity features of Zfs make it very attractive too. I fully expect
>> things will improve too with Zfs.
>>
>> If you want a lot of certainty in your choices, you may want to
>> consult various vendors if lustre systems.
>>
>> Scott
>>
>>
>>
>>
>> On June 8, 2014 11:42:15 AM CDT, "Dilger, Andreas"
>> <andreas.dilger at intel.com<mailto:andreas.dilger at intel.com>> wrote:
>>
>> Scrub and resilver have nothing to so with defrag.
>>
>> Scrub is scanning of all the data blocks in the pool to verify
>> their checksums and parity to detect silent data corruption, and
>> rewrite the bad blocks if necessary.
>>
>> Resilver is reconstructing a failed disk onto a new disk using
>> parity or mirror copies of all the blocks on the failed disk. This is
>> similar to scrub.
>>
>> Both scrub and resilver can be done online, though resilver of
>> course requires a spare disk to rebuild onto, which may not be
>> possible to add to a running system if your hardware does not support
>> it.
>>
>> Both of them do not "improve" the performance or layout of data on
>> disk. They do impact performance because they cause a lot if random
>> IO to the disks, though this impact can be limited by tunables on the
>> pool.
>>
>> Cheers, Andreas
>>
>> On Jun 8, 2014, at 4:21, "Sean Brisbane"
>> <s.brisbane1 at physics.ox.ac.uk<mailto:s.brisbane1 at physics.ox.ac.uk><mailto:s.brisbane1 at physics.ox.ac.uk>>
>> wrote:
>>
>> Hi Scott,
>>
>> We are considering running zfs backed lustre and the factor of
>> 10ish performance hit you see worries me. I know zfs can splurge bits
>> of files all over the place by design. The oracle docs do recommend
>> scrubbing the volumes and keeping usage below 80% for maintenance and
>> performance reasons, I'm going to call it 'defrag' but I'm sure
>> someone who knows better will probably correct me as to why it is not
>> the same.
>> So are these performance issues after scubbing and is it possible
>> to scrub online - I.e. some reasonable level of performance is
>> maintained while the scrub happens?
>> Resilvering is also recommended. Not sure if that is for
>> performance reasons.
>>
>> http://docs.oracle.com/cd/E23824_01/html/821-1448/zfspools-4.html
>>
>>
>>
>> Sent from my HTC Desire C on Three
>>
>> ----- Reply message -----
>> From: "Scott Nolin"
>> <scott.nolin at ssec.wisc.edu<mailto:scott.nolin at ssec.wisc.edu><mailto:scott.nolin at ssec.wisc.edu>>
>> To: "Anjana Kar"
>> <kar at psc.edu<mailto:kar at psc.edu><mailto:kar at psc.edu>>,
>> "lustre-discuss at lists.lustre.org<mailto:lustre-discuss at lists.lustre.org><mailto:lustre-discuss at lists.lustre.org>"
>> <lustre-discuss at lists.lustre.org<mailto:lustre-discuss at lists.lustre.org><mailto:lustre-discuss at lists.lustre.org>>
>>
>> Subject: [Lustre-discuss] number of inodes in zfs MDT
>> Date: Fri, Jun 6, 2014 3:23 AM
>>
>>
>>
>> Looking at some of our existing zfs filesystems, we have a couple
>> with zfs mdts
>>
>> One has 103M inodes and uses 152G of MDT space, another 12M and
>> 19G. I’d plan for less than that I guess as Mr. Dilger suggests. It
>> all depends on your expected average file size and number of files
>> for what will work.
>>
>> We have run into some unpleasant surprises with zfs for the MDT, I
>> believe mostly documented in bug reports, or at least hinted at.
>>
>> A serious issue we have is performance of the zfs arc cache over
>> time. This is something we didn’t see in early testing, but with
>> enough use it grinds things to a crawl. I believe this may be
>> addressed in the newer version of ZFS, which we’re hopefully awaiting.
>>
>> Another thing we’ve seen, which is mysterious to me is this it
>> appears hat as the MDT begins to fill up file create rates go down.
>> We don’t really have a strong handle on this (not enough for a bug
>> report I think), but we see this:
>>
>>
>> 1.
>> The aforementioned 104M inode / 152GB MDT system has 4 SAS drives
>> raid10. On initial testing file creates were about 2500 to 3000 IOPs
>> per second. Follow up testing in it’s current state (about half
>> full..) shows them at about 500 IOPs now, but with a few iterations
>> of mdtest those IOPs plummet quickly to unbearable levels (like 30…).
>> 2.
>> We took a snapshot of the filesystem and sent it to the backup MDS,
>> this time with the MDT built on 4 SAS drives in a raid0 - really not
>> for performance so much as “extra headroom” if that makes any sense.
>> Testing this the IOPs started higher, at maybe 800 or 1000 (this is
>> from memory, I don’t have my data in front of me). That initial
>> faster speed could just be writing to 4 spindles I suppose, but
>> surprising to me, the performance degraded at a slower rate. It took
>> much longer to get painfully slow. It still got there. The
>> performance didn’t degrade at the same rate if that makes sense - the
>> same number of writes on the smaller/slower mdt degraded the
>> performance more quickly. My guess is that had something to do with
>> the total space available. Who knows. I believe restarting lustre
>> (and certainly rebooting) ‘resets the clock’ on the file create
>> performance degradation.
>>
>> For that problem we’re just going to try adding 4 SSD’s, but it’s
>> an ugly problem. Also are once again hopeful new zfs version
>> addresses it.
>>
>> And finally, we’ve got a real concern with snapshot backups of the
>> MDT that my colleague posted about - the problem we see manifests in
>> essentially a read-only recovered file system, so it’s a concern and
>> not quite terrifying.
>>
>> All in all, the next lustre file system we bring up (in a couple
>> weeks) we are very strongly considering going with ldiskfs for the
>> MDT this time.
>>
>> Scott
>>
>>
>>
>>
>>
>>
>>
>>
>> From: Anjana Kar<mailto:kar at psc.edu>
>> Sent: Tuesday, June 3, 2014 7:38 PM
>> To:lustre-discuss at lists.lustre.org<mailto:discuss at lists.lustre.org><mailto:lustre-discuss at lists.lustre.org>
>>
>> Is there a way to set the number of inodes for zfs MDT?
>>
>> I've tried using --mkfsoptions="-N value" mentioned in lustre 2.0
>> manual, but it
>> fails to accept it. We are mirroring 2 80GB SSDs for the MDT, but the
>> number of
>> inodes is getting set to 7 million, which is not enough for a 100TB
>> filesystem.
>>
>> Thanks in advance.
>>
>> -Anjana Kar
>> Pittsburgh Supercomputing Center
>> kar at psc.edu<mailto:kar at psc.edu><mailto:kar at psc.edu>
>> ------------------------------------------------------------------------
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