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Re: mkfs.xfs error creating large agcount an raid

To: Paul Anderson <pha@xxxxxxxxx>
Subject: Re: mkfs.xfs error creating large agcount an raid
From: Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx>
Date: Mon, 27 Jun 2011 15:55:26 -0500
Cc: Eric Sandeen <sandeen@xxxxxxxxxxx>, linux-xfs@xxxxxxxxxxx, Marcus Pereira <marcus@xxxxxxxxxxx>
In-reply-to: <BANLkTikJe7ayzwD2Yqc7BHePfZ4x-M_SyQ@xxxxxxxxxxxxxx>
References: <4E063BC6.9000801@xxxxxxxxxxx> <4E0694CC.8050003@xxxxxxxxxxxxxxxxx> <4E06C967.2060107@xxxxxxxxxxx> <20110626235959.GC32466@dastard> <4E07FA07.4050907@xxxxxxxxxxxxxxxxx> <4E0803AA.20809@xxxxxxxxxxx> <4E08456F.1090503@xxxxxxxxxxxxxxxxx> <BANLkTimJm5Fe1LvD1AQYZC5QCDs0gXJpFA@xxxxxxxxxxxxxx> <4E089D4E.1060503@xxxxxxxxxxx> <BANLkTikJe7ayzwD2Yqc7BHePfZ4x-M_SyQ@xxxxxxxxxxxxxx>
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On 6/27/2011 10:27 AM, Paul Anderson wrote:

> There is nothing in the man page I see indicating what is good or bad
> regarding allocation groups - either document it there or warn in the
> software.  If allocation algorithms are linear with respect to
> allocation groups, the something like this should be stated in the man
> pages.

It is.  From the 2nd paragraph of 'man 5 xfs':

...The data section is divided into a number of allocation groups. The
number and size of the allocation groups are chosen by mkfs.xfs(8) so
that there is normally a small number of equal-sized groups. The number
of allocation groups controls the amount of parallelism available in
file and block allocation. It should be increased from the default if
there is sufficient memory and a lot of allocation activity. The number
of allocation groups should not be set very high, since this can cause
large amounts of CPU time to be used by the filesystem, especially when
the filesystem is nearly full. More allocation groups are added (of the
original size) when xfs_growfs(8) is run.

Maybe some of this information could/should be moved to the agcount
section of 'man mkfs.xfs'.  I'll concede that "should not be set very
high" is subjective for novice XFS users.  To Marcus 20,000 may not be
"very high". :)

-- 
Stan

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