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
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