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Question about Number of Allocation Groups

To: xfs@xxxxxxxxxxx
Subject: Question about Number of Allocation Groups
From: Gim Leong Chin <chingimleong@xxxxxxxxxxxx>
Date: Tue, 31 May 2011 13:30:08 +0800 (SGT)
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I have a question about the number of allocation groups in XFS.

In SUSE Linux Enterprise 10 SP2 kernel 2.6.16, the default number of allocation 
groups from what I have seen is 16 for a 144 GB filesystem, 32 for a 1.1 TB 

In SUSE Linux Enterprise 11 kernel 2.6.27, the default number of allocation 
groups is 4 even for a 800 GB filesystem (from what I have observed on openSUSE 
11.4 kernel 2.6.37, that number is still 4).  I have observed that for XFS on 
mdraid the default is 16 for a 101 GB filesytem, 32 for a 4.5 TB filesystem.

1) May I know what is the  method for computing the default number of 
allocation groups, and why the big change from kernel 2.6.16 to 2.6.27 (16, 32 
to 4)?

2) What are the guide lines for deciding how many allocation groups we should 
specify?  Is the number related to filesystem size, the IO bandwidth, the 
number of processor cores, the number of parallelism threads we want to occupy 
the system, system usage/workload characteristics?

3) I am sure there is some point for any system where an increasing number of 
allocation groups will first increase performance, and then the performance 
will start to drop?

4) What would be an optimum number for the root filesystem for the operating 
system?  As I have 100 GB for the "/", my OS partitions not on mdraid have the 
default 4 allocation groups for kernel 2.6.27 and 2.6.37.

Thank you!  This are questions that have been puzzling me for a while I would 
really appreciate the answers.

Chin Gim Leong

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