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SubjectRe: tuning for large files in xfs
here are the results:

under 2.6.8 kernel with agsize=2g (440 some AGs): 6.9ms avg access
under 2.6.8 kernel with agcount=32: 6.2ms
under 2.6.17 kernel with partition made in 2.6.8 with agcount=32: 6.2ms
under 2.6.17 kernel just reading from /dev/sdb1: 6.2ms
under 2.6.17 kernel with new partition (made under 2.6.17) with
agcount=32, file created via the xfs_io reserve call: 6.9 ms
under 2.6.17 kernel just reading from /dev/sdb1: 6.9ms (not sure why
this changed from 6.2 the day before)...

So it seems like going to 32 AGs helped about 10%, but other then that
not much else is making much of a difference... now I am moving on and
trying to break the RAID up and testing individual disks to see their
performance...

Nathan Scott wrote:
> On Tue, May 23, 2006 at 06:41:36PM -0700, fitzboy wrote:
>
>>I read online in multiple places that the largest allocation groups
>>should get is 4g,
>
>
> Thats not correct (for a few years now).
>
>
>>I was also thinking that the more AGs the better since I do a lot of
>>parallel reads/writes... granted it doesn't change the file system all
>>that much (the file only grows or get existing blocks get modified), so
>>I am not sure if the number of AGs matter, does it?
>
>
> Yes, it can matter. For large extents like you have here, AGs
> introduce a discontinuity that you'd otherwise not have.
>
>
>>Sorry, I meant that moving the Inode size to 2k (over 256bytes) gave me
>>a sizeable increase in performance... I assume that is because the
>>extent map can be smaller now (since blocks are much larger, less blocks
>>to keep track of). Of course, ideal would be to have InodeSize be large
>>and blocksize to be 32k... but I hit the limits on both...
>
>
> It means that more extents/btree records fit inline in the inode,
> as theres more space available after the stat data. 2k is your
> best choice for inode size, stick with that.
>
>
>>>- Preallocate the space in the file - i.e. before running the
>>>dd you can do an "xfs_io -c 'resvsp 0 2t' /mnt/array/disk1/xxx"
>>>(preallocates 2 terabytes) and then overwrite that. Yhis will
>>>give you an optimal layout.
>>
>>I tried this a couple of times, but it seemed to wedge the machine... I
>>would do: 1) touch a file (just to create it), 2) do the above command
>
>
> Oh, use the -f (create) option and you won't need a touch.
>
>
>>which would then show effect in du, but the file size was still 0 3) I
>>then opened that file (without O_TRUNC or O_APPEND) and started to write
>>out to it. It would work fine for a few minutes but after about 5 or 7GB
>>the machine would freeze... nothing in syslog, only a brief message on
>>console about come cpu state being bad...
>
>
> Hmm - I'd be interested to hear if that happens with a recent
> kernel.
>
>
>>>- Your extent map is fairly large, the 2.6.17 kernel will have
>>>some improvements in the way the memory management is done here
>>>which may help you a bit too.
>>
>>we have plenty of memory on the machines, shouldn't be an issue... I am
>>a little cautious about moving to a new kernel though...
>
>
> Its not the amount of memory that was the issue here, its more the
> way we were using it that was a problem for kernels of the vintage
> you're using here. You will definately see better performance in
> a 2.6.17 kernel with that large extent map.
>
> cheers.
>

--
Timothy Fitz
Lead Programmer

iParadigms, LLC
1624 Franklin St., 7th Floor
Oakland, CA 94612

p. +1-510-287-9720 x233
f. +1-510-444-1952
e. fitzboy@iparadigms.com

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