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Re: Seeking XFS tuning advice for PostgreSQL on SATA SSDs/Linux-md

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: Seeking XFS tuning advice for PostgreSQL on SATA SSDs/Linux-md
From: Johannes Truschnigg <johannes.truschnigg@xxxxxxxxxxx>
Date: Wed, 16 Apr 2014 10:21:44 +0200
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140415213408.GL15995@dastard>
References: <534D24AB.1050605@xxxxxxxxxxx> <20140415213408.GL15995@dastard>
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Hi Dave,

On 04/15/2014 11:34 PM, Dave Chinner wrote:
On Tue, Apr 15, 2014 at 02:23:07PM +0200, Johannes Truschnigg wrote:
Hi list,
[...]
o Intel C606-based Dual 4-Port SATA/SAS HBA (PCIID 8086:1d68)

How much write cache does this have?

It's a plain HBA; it doesn't have write cache (or a BBU) of its own.


o 6x Samsung 830 SSD with 512GB each, 25% reserved for HPA

830? That's the previous generation of drives - do you mean 840?

No, I really mean 830 - we've tested 840 EVO as well, and they performed quite well, too, however from what I've seen on the web the longevity of Samsung's TLC flash choice in 840 disks isn't as promising as those of the 830s MLC variant. We might be switching over to 840 EVO or one of their successors once the 830s wear out, or we need to expand capacity, but we do have a number of 830s in stock that we'll use first.


When benchmarking the individual SSDs with fio (using the libaio
backend), the IOPS we've seen were in the 30k-35k range overall for
4K block sizes.

They don't sustain that performance over 20+ minutes of constant IO,
though. Even if you have 840s (I have 840 EVOs in my test rig), the
sustained performance of 4k random write IOPS is somewhere around
4-6k each. See, for example, the performance consistency graphs here:

http://www.anandtech.com/show/7173/samsung-ssd-840-evo-review-120gb-250gb-500gb-750gb-1tb-models-tested/6

Especially the last one that shows a zoomed view of the steady state
behaviour between 1400s and 2000s of constant load.

I used tkperf[0] to benchmark the devices, both on Intel's SAS HBA and on a LSI 2108 SAS RAID-Controller. I did runs for the 512GB 830 with 25% over-provisioning, and runs for 1TB 840 EVO with 0% op and 25% op (two different disks with the same firmware). tkperf tries hard to achieve steady state by torturing the devices for a few hours before the actual benchmarking takes place, and will only do so after that steady state has been reached.

From what I've seen, the over-provisioning is absolutely crucial to get anywhere near acceptable performance; since Anandtech doesn't seem to use it, I'll trust my tests more.

For reference: the 750GB usable-space EVO clocked in at ~35k 4k IOPS on the LSI 2108, whilst the 1000GB usable-space sister disk still hasn't finished the benchmark run, because it's _so much slower_. The benchmark was started about ten days ago for both disks; the 750GB disk finished after some 2 or 3 days, and I'm _still_ waiting for the 1000GB disk to finish benchmarking. Only then I'll be able to look at the pretty graphs and tables tkperf generates, but when tailing the log and watching iostat, I can already draw some early conclusions as to how these two configurations perform, and they're not in the same ballpark at all.


The 830 series are old enough that they were reviewed before this
was considered an important metric for SSD comparison, and so there
is no equivalent information available for them. However, they are
likely to be significantly slower and less deterministic in their
behaviour than the 840s under the same load...

Afaik, 840 EVO's relatively high peak performance stems from the DRAM buffer these disks supposedly have built in, while the 830 lacks that kind of trick. Given that the EVO's performance drops after that buffer has worked its magic, I'd actually expect the 830 to perform _more consistent_ (not necessarily better, even on average, though) than the 840 EVO. We'll see if that holds true if/when we put 840 EVOs into service, I guess.


The host will be on the receiving end of a pg9.0
streaming replication cluster setup where the master handles ~50k
IOPS peak, and I'm thinking what'd be a good approach to design the
local storage stack (with availability in mind) in a way that has a
chance to keep up with our flash-based FC SAN.

I'd be surprised if it can keep up after a couple of months of
production level IO going to the SSDs...

Yeah, that remains to be seen, and it'll be very interesting - if anyone's interested, I'll be happy to share our learnings from this project once we have enough data worth talking about. Remember, the numbers I posted are _peak_ load at the master though, most of the time, we don't exceed 10k IOPS, and some of the time, the system is practically idle. That might give the SSD controllers enough time to work their garbage collection secret sauce magic, and sustain high(er) performance over most of their lifetimes.


After digging through linux-raid archives, I think the most sensible
approach are two-disk pairs in RAID1 that are concatenated via
either LVM2 or md (leaning towards the latter, since I'd expect that
to have a tad less overhead),

I'd stripe them (i.e. RAID10), not concantenate them so as to load
both RAID1 legs evenly.

Afaik, the problem with md is that each array (I'm pretty convinced that also holds true for RAID10, but I'm not 100% sure) only has one associated kernel thread for writes, which should make that kind of setup worse, at least in theory and in terms of achiveable parallelism, than the setup I described. I'd be very happy to see a comparison between the two setups for high-IOPS devices, but I haven't yet found one anywhere.


> [...]
I've experimented with mkfs.xfs (on top of LVM only; I don't know if
it takes into account lower block layers and seen that it supposedly
chooses to default to an agcount of 4, which seems insufficient
given the max. bandwidth our setup should be able to provide.

The number of AGs has no bearing on acheivable bandwidth. The number
of AGs affects allocation concurrency. Hence if you have 24 CPU
cores, I'd expect that you want 32 AGs. Normally with a RAID array
this will be the default, but it seems that RAID1 is not triggering
the "optimise for allocation concurrency" heuristics in mkfs....

Thanks, that is a very useful heads-up! What's the formula used to get to 32 AGs for 24 CPUs - just (num_cpus * 4/3), and is there a simple explanation for why this is an ideal starting point? And is that an advisable rule of thumb for xfs in general?


Apart from that, is there any kind of advice you can share for
tuning xfs to run postgres (9.0 initially, but we're planning to
upgrade to 9.3 or later eventually) on in 2014, especially
performance-wise?

Apart from the AG count and perhaps tuning the sunit/swidth to match
the RAID0 part of the equation, I wouldn't touch a thing unless you
know that there's a problem that needs fixing and you know exactly
what knob will fix the problem you have...

OK, I'll read up on stripe width impact and will (hopefully) have enough time to test a number of configs that should make sense.

Many thanks for your contribution and advice! :)

[0]: http://www.thomas-krenn.com/en/oss/tkperf.html

--
Mit freundlichen Grüßen
Johannes Truschnigg
Senior System Administrator
--
mailto:johannes.truschnigg@xxxxxxxxxxx (in dringenden Fällen bitte an info@xxxxxxxxxxx)

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