Re: allocsize mount option behavior

[Dave Chinner <david@fromorbit.com>]

On Tue, Nov 16, 2010 at 04:56:00PM -0500, Ivan.Novick@emc.com wrote:
> Hello,
> 
> 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.

Expected behaviour.

> 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.

The latter - it's an in-memory, per-inode speculative allocation
that is removed when the file is closed.

> 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?

Yes, exactly.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

_______________________________________________
xfs mailing list
xfs@oss.sgi.com
http://oss.sgi.com/mailman/listinfo/xfs