Re: Warning: AG size is a multiple of stripe width?

[Eric Sandeen <sandeen@sandeen.net>]

On 6/17/11 6:36 AM, Gim Leong Chin wrote:
> Hi,
> 
> 
> I have a Sun workstation with eight Cheetah 15K.5 SAS 300 GB on RAID
> 1E (RAID 10) on LSI SAS3081E-R.
> 
> I am installing SLED 11 SP1 on it and I thought I will do a thorough
> optimization right down to the partition boundaries.
> 
> Since the default for XFS is to create four aggregation groups, and
> with the reasoning that Cheetah can do double the seeks of normal
> 7200 RPM drives, I have four aggregation groups per drive for a total

"Allocation groups" just FWIW :)

Probably no real reason to try to outfox the defaults by doubling
AGs though, at least at this point.

> of 16 for 70 GB /dev/sda2 partition, and eight per drive for total of
> 32 for /dev/sda3 partition (1011 GB).
> 
> I have aligned the partition start and end with the stripe width
> boundaries. The stripe size is 64 kB, stripe width is 4*64 kB = 256
> kB, in terms of 512 byte sectors:

> 
> 70 GB /
> 
> No      Start           End             Number
> 1       512             67109375        32 GB = 67108864 sectors = 131072 stripe sets
> 
> 2       67109376        213910015       70 GB = 146800640 sectors = 286720 stripe sets
> 
> 3       213910016       2335932415      Left = 2335932416 - 213910016 = 2122022400 sectors = 4144575 stripe sets
> 
> 
> When I do the following:
> 
> mkfs.xfs -f -b size=4k -d agcount=16,su=64k,sw=4 -i size=256,align=1,attr=2 -l version=2,su=64k,lazy-count=1 -n version=2 -s size=512 -L / /dev/sda2

You are restating many defaults here, I'm not sure why...

I would probably just drop the agcount specification and let mkfs do its own thing here; left to its own devices it will choose 4 AGs.

> Warning: AG size is a multiple of stripe width. This can cause
> performance problems by aligning all AGs on the same disk. To avoid
> this, run mkfs with an AG size that is one stripe unit smaller, for
> example 1146864

mkfs.xfs -f -b size=4k -d agsize=1146864b,su=64k,sw=4  ...

does work too, if you really want that many AGs for some reason:

meta-data=testfile               isize=256    agcount=16, agsize=1146864 blks
         =                       sectsz=512   attr=2
data     =                       bsize=4096   blocks=18349824, imaxpct=25
         =                       sunit=16     swidth=64 blks
naming   =version 2              bsize=4096   ascii-ci=0
log      =internal log           bsize=4096   blocks=8960, version=2
         =                       sectsz=512   sunit=16 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0

-Eric

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