[lustre-discuss] Tuning for metadata performance

Vicker, Darby J. (JSC-EG111)[Jacobs Technology, Inc.] darby.vicker-1 at nasa.gov
Fri Jan 8 10:35:19 PST 2021

Perhaps a better question to ask (although very closely related) would be how can we improve the MD tests in the io500 benchmark?

In the info below this is the info on these file systems:

nobackup – a lustre FS on the hardware we've been discussing with a ZFS MDT, nominally running on mds0
ephemeral – a lustre FS on the hardware we've been discussing with an ldiskfs MDT, nominally running on mds1
scratch – a standard NFS mount
local – a local SSD

A little more background on the motivation here.  We have some fairly large software development projects in the lab.  One of the largest active projects has a git repo with about 500,000 files totaling 5 GB in size. A clone of these repo takes 550 seconds on lustre and about 150 seconds on NFS.  A status takes 15 seconds on lustre and 3 seconds on NFS.  Not surprisingly, the timings are greatly reduced on a local SSD.  See the attached plot in git_timings.pdf for details.  The slowness on lustre is largely (completely?) driven by the MD performance.  Obviously, we work with the repo on a local file system when possible to avoid the performance hit.  But one of the workflows involves Monte Carlo analysis against this repo, varying dozens of parameters, running 1000's of cases and analyzing the results.  This produces a lot of data and necessitates the shared FS for both running the Monte Carlo cases and simply storing the amounts of data these runs produce.

There are several other scenarios in which we are working with smaller, but still sizeable, data sets (git repos and other forms) on the lustre file system and the MD sluggishness is noticeable and annoying.  So we would like to try and improve MD performance.

To further characterize and compare the IO performance on these file systems, I've run the io500 benchmarks.  The attached plots show the results.  This is a completely "out of the box" run on a single node.  That is, I'm just running "./io500.sh config-minimal.ini".  (I've run the 10-node results too (or tried to) for more direct comparison to the results on io500.org but that's a slightly different objective.)  I figure the single node run is analogous to a person working with a git repo scenario.  This is on a 10 gigabit ethernet client.  Details attached but the MD results are fairly consistent with the above git timings – lustre is about 3x to 10x slower than NFS.  I'd be curious to get some feedback on these MD performance numbers.  Do they seem low compared to other LFS's out there?  As I mentioned in the original post in this thread, our numbers are quite low when compared to even the lowest numbers on the current io500 list.

How is MD performance expected to increase with increasing numbers of clients?  I know bandwidth increases as you grab more OST' but would MD performance be expected to increase at all?  We are not using DoM or DNE.

Also as mentioned before, we will upgrade lustre soon.  I'd like to stick with the 2.12 LTS stream.  But would the upcoming 2.14 have any potential MD performance advantages?

From: lustre-discuss <lustre-discuss-bounces at lists.lustre.org> on behalf of "Vicker, Darby J. (JSC-EG111)[Jacobs Technology, Inc.]" <darby.vicker-1 at nasa.gov>
Date: Wednesday, January 6, 2021 at 9:29 AM
To: Andreas Dilger <adilger at whamcloud.com>
Cc: "lustre-discuss at lists.lustre.org" <lustre-discuss at lists.lustre.org>
Subject: Re: [lustre-discuss] [EXTERNAL] Re: Tuning for metadata performance

My apologies – I posted some bad info.  While we started out with the HDD's in the MDS, pretty early on we switched to SSD's.  So that's not the source of our MD slowness.  Can you do NVMe in an external JBOD?

From: Andreas Dilger <adilger at whamcloud.com>
Date: Tuesday, January 5, 2021 at 11:51 AM
To: "Vicker, Darby J. (JSC-EG111)[Jacobs Technology, Inc.]" <darby.vicker-1 at nasa.gov>
Cc: "lustre-discuss at lists.lustre.org" <lustre-discuss at lists.lustre.org>
Subject: [EXTERNAL] Re: [lustre-discuss] Tuning for metadata performance

Probably the best single thing you could do for metadata performance
would be to switch to SSD, or better NVMe, storage.  ZFS is very sync
and IOPS hungry, so using HDDs is killer for ZFS metadata performance.

If you want to minimize the downtime, you could incrementally replace the
HDDs in the zpool with larger SSD devices and resilver between each
one.  I recall LLNL doing this in the first months of their first ZFS-based
Lustre filesystem for this reason.

Going to NVMe-based devices is even better for IOPS/bandwidth, but
can't be done completely live.  You could potentially use repeated zfs
send/recv to get an almost uptodate copy on a new MDS, then take a small
outage to do the final resync. However, I've also seen reports that send/recv is painfully slow with HDD MDTs so you should probably test that before committing to a solution.

Cheers, Andreas

On Jan 5, 2021, at 08:47, Vicker, Darby J. (JSC-EG111)[Jacobs Technology, Inc.] <darby.vicker-1 at nasa.gov> wrote:

I'm looking for some advice on tuning our existing lustre file system to achieve better metadata performance.  This file system is getting fairly old – its been in production for almost 4 years now.  The hardware and our existing tuning efforts can be found here.


The hardware is the same but we have upgraded the software stack a few times – now on CentOS 7.6, ZFS 0.7.9 and lustre 2.10.8.  We do plan to upgrade to the latest CentOS 7.x and either lustre 2.12 or 2.13 soon.  The MDS hardware isn't well-described in that thread so here are more details:

Chassis: Supermicro 2U Twin Server
Processor: 4 x Quad­Core Xeon Processor E5­2637 v2 3.50GHz (2 sockets/8 cores per node)
Memory: 16 x 16GB PC3­14900 1866MHz DDR3 ECC Registered DIMM (128GB per node)

External JBOD:
Chassis: 24x Hot­Swap 2.5" SAS ­ 12Gb/s SAS Dual Expander
Drives: 12 x 600GB SAS 3.0 12.0Gb/s 15000RPM ­ 2.5" ­ Seagate Enterprise Performance 15K HDD (512n)
Controller Card: LSI SAS 9300-8e SAS 12Gb/s PCIe 3.0 8-Port Host Bus Adapter

The above hardware and tuning served us well for a long time but the lab has grown, both in number of lustre clients (now up to ~200 ethernet clients and ~500 IB clients) and the number of users in the lab.  With the extra users have come different types of workloads.  Peviously, the file system was most used for workloads with a fairly small number of large files.  We now see workloads that include 100's of concurrent processes all doing mixed small and large file IO on a lot of files (e.g. each process clones a repo, compiles a code and runs a serial sim that writes a lot of data).

I recently ran the io500 tests and our LFS stats for MDEasy and MDHard are pretty bad, even when compared to the lowest MD stats on the current io500 list.  Our standard NFS server handily beats our LFS wrt MD performance.  So I'm hopeful that we can squeeze more MD performance out of our LFS.  Obviously, software tuning on the existing hardware would be preferred but we are open to hardware additions/upgrades if that would help (e.g. adding more MDS's).  There are a lot of tuning options in both ZFS and lustre so I'm hoping someone can point me in the right direction.  Are DNE and/or DoM expected to help?  I attended the SC20 Lustre BoF and it sounds like 2.13 has some metadata performance improvements, so just an upgrade might help.  We have dual MDS's now but for HA, not performance.  I'd hate to lose the HA aspect as we utilize it for failover quite a bit (maintenance, etc.) but it would probably be worth it if MD performance was significantly improved.  If I understand correctly, there is some overhead with DNE and performance suffers with just two MDS's with a benefit with 4 or more MDS's, correct?  So that wouldn't be a good option for us unless we add MDS's?  Would an upgrade to SSD or NVMe in our MDTs help?

I would greatly appreciate thoughts on the best path forward for making improvements.

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