allocsize mount option

[Gim Leong Chin <chingimleong@xxxxxxxxxxxx>]

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


      New Email names for you! 
Get the Email name you've always wanted on the new @ymail and @rocketmail. 
Hurry before someone else does!
http://mail.promotions.yahoo.com/newdomains/sg/