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allocsize mount option

To: xfs@xxxxxxxxxxx
Subject: allocsize mount option
From: Gim Leong Chin <chingimleong@xxxxxxxxxxxx>
Date: Tue, 12 Jan 2010 01:25:14 +0800 (SGT)
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Hi,

Mount options for xfs
       allocsize=size
       Sets  the buffered I/O end-of-file preallocation size when doing delayed 
allocation writeout (default size is 64KiB). 


I read that setting allocsize to a big value can be used to combat filesystem 
fragmentation when writing big files.

I do not understand how allocsize works.  Say I set allocsize=1g, but my file 
size is only 1 MB or even smaller.  Will the rest of the 1 GB file extent be 
allocated, resulting in wasted space and even file fragmentation problem?

Does setting allocsize to a big value result in performance gain when writing 
big files?  Is performance hurt by a big value setting when writing files 
smaller than the allocsize value?

I am setting up a system for HPC, where two different applications have 
different file size characteristics, one writes files of GBs and even 128 GB, 
the other is in MBs to tens of MBs.

I am not able to find documentation on the behaviour of allocsize mount option.

Thank you.


Chin Gim Leong


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