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question on agcount and agsize

To: xfs@xxxxxxxxxxx
Subject: question on agcount and agsize
From: Josh Greenberg <josh@xxxxxxxxxxx>
Date: Tue, 19 May 2009 10:11:33 -0400
User-agent: Thunderbird (X11/20090324)
I've just installed xfs for the first time and I'm trying to put an xfs file system on a large disk. When I just tried with no arguments, it told me that the agsize was too large and the maximum is 268435455 blocks. I wasn't familiar with allocation groups at all so I researched this a little bit. I found out what an allocation group is but there aren't many recommendations on agcount other than "don't make it too big" because it slows the disk down as it gets full. However, after playing with the numbers, I found that the lowest number it will accept for agcount is 44. I now have a file system with the following structure:

meta-data=/dev/md0       isize=256    agcount=44, agsize=266361205 blks
        =                       sectsz=512
data     =                       bsize=4096   blocks=11719892992, imaxpct=25
        =                       sunit=0      swidth=0 blks, unwritten=1
naming   =version 2              bsize=4096
log      =internal log           bsize=4096   blocks=32768, version=1
        =                       sectsz=512   sunit=0 blks
realtime =none                   extsz=65536  blocks=0, rtextents=0

I have two questions:
1. Does the agcount have to be a power of 2? Almost every example of agcount I've seen online has been a power of 2 (2, 4, 8, 16, and 32). I originally tried an agcount of 64 which worked but then tried to size it down so it wasn't too large. Will this cause problems later. 2. Is 44 too large? Will I experience a slow down as the disk fills? I've seen people recommend that you not go over agcount=32 and even someone who said not to exceed agcount=8. What are the long term ramifications of this?



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