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allocsize mount option

To: <xfs@xxxxxxxxxxx>
Subject: allocsize mount option
From: <Ivan.Novick@xxxxxxx>
Date: Tue, 28 Sep 2010 14:53:46 -0400
Accept-language: en-US
Acceptlanguage: en-US
Cc: <Timothy.Heath@xxxxxxx>
Thread-index: ActfPnqegT+QQtiQekKP4yd2/QdcUA==
Thread-topic: allocsize mount option
Hi all,

According to the documentation the allocsize mount option: "Sets the
buffered I/O end-of-file preallocation size when doing delayed allocation
writeout"

Will this value limit "extent" sizes to be be no smaller than the allocsize?

I have set the following mount options:
(rw,noatime,nodiratime,logbufs=8,allocsize=512m)

And yet, depending on the workload, the extent sizes are often 1 or 2 orders
of magnitude lower than 512 MB ...

If I wanted to do further reading on the subject, can someone point me to an
approximate location in the code where the size of a newly created extent is
determined?

Cheers,
Ivan Novick

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