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specify agsize?

To: xfs@xxxxxxxxxxx
Subject: specify agsize?
From: aurfalien <aurfalien@xxxxxxxxx>
Date: Sat, 13 Jul 2013 17:11:38 -0700
Delivered-to: xfs@xxxxxxxxxxx
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Hello again,

I have a Raid 6 x16 disk array with 128k stripe size and a 512 byte block size.

So I do;

mkfs.xfs -f -l size=512m -d su=128k,sw=14 /dev/mapper/vg_doofus_data-lv_data

And I get;

meta-data=/dev/mapper/vg_doofus_data-lv_data isize=256    agcount=32, 
agsize=209428640 blks
         =                       sectsz=512   attr=2, projid32bit=0
data     =                       bsize=4096   blocks=6701716480, imaxpct=5
         =                       sunit=32     swidth=448 blks
naming   =version 2              bsize=4096   ascii-ci=0
log      =internal log           bsize=4096   blocks=131072, version=2
         =                       sectsz=512   sunit=32 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0


All is fine but I was recently made aware of tweaking agsize.  So I would like 
to mess around and iozone any diffs between the above agcount of 32 and 
whatever agcount changes I may do.

I didn't see any mention of agsize/agcount on the XFS FAQ and would like to 
know, based on the above, why does XFS think I have 32 allocation groups with 
the corresponding size?  And are these optimal numbers?

Thanks in advance,

- aurf

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