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xfs preallocation timeout

To: xfs@xxxxxxxxxxx
Subject: xfs preallocation timeout
From: Michael Monnerie <michael.monnerie@xxxxxxxxxxxxxxxxxxx>
Date: Wed, 11 Feb 2009 15:22:11 +0100
Organization: it-management http://it-management.at
User-agent: KMail/1.10.3 (Linux/2.6.27.10-ZMI; KDE/4.1.3; x86_64; ; )
Does /proc/sys/vm/dirty_expire_centisecs influence the maximum time XFS 
takes before the "in-memory but not on disk" preallocation actually 
starts writing to disk? If not, which parameter does influence how long 
files are grouped together in memory before writing them really to disk? 
The longer the better for performance, but more risk for data loss of 
course. 

I ask this because I want to know when copying files over the net to an 
XFS drive, how I can prevent fragmentation to occur. Example: If a 
packet of 1024 Bytes arrives every 1/100th second, it needs 0.64s to get 
64KB of data, 1.28s to get 128KB. If the prealloc timeout is one second, 
it's effectively not used in this case. So if you got a server where you 
know files arrive at a certain speed, fine tuning this prealloc timeout 
could help prevent fragmentation.

mfg zmi
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