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allocsize mount option behavior

To: <xfs@xxxxxxxxxxx>
Subject: allocsize mount option behavior
From: <Ivan.Novick@xxxxxxx>
Date: Tue, 16 Nov 2010 16:56:00 -0500
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Cc: <Timothy.Heath@xxxxxxx>
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Thread-topic: allocsize mount option behavior
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I am using the allocsize mount option to reduce fragmentation, by ensuring 
extents are of a minimum size.

Under a workload with heavy concurrency we noticed that disk space usage went 
up noticeably.

Can someone clarify the expected behavior of allocsize.  Does it force files to 
be at least the allocsize size on disk, or is the allocsize size reserved for 
each open file and then freed up if it is not used and the file is closed, or 
does it work some other way.

For example if allocsize=1g and I open 1000 files and start writing a small 
amount of data to each file …. Would the device show as using 1000* 1g of disk 
space while the files are opened and after they are closed then only the amount 
of space actually written by the user would be shown as used on disk?

Ivan Novick

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