2 question about XFS fragmentation and _fsr: Q2

[Janos Haar]

----- Original Message ----- 
From: "Christoph Hellwig" <hch at infradead.org>
To: "Dave Chinner" <david at fromorbit.com>
Cc: "Janos Haar" <janos.haar at netcenter.hu>; <linux-xfs at oss.sgi.com>
Sent: Tuesday, April 12, 2011 1:37 AM
Subject: Re: 2 question about XFS fragmentation and _fsr


> On Tue, Apr 12, 2011 at 07:42:38AM +1000, Dave Chinner wrote:
>> Depends on your samba configuration to how it does writes. If it is
>> conigured to do sync writes then it will have all sorts of
>> fragmentation problems. So without more details about the workload,
>> the kernel that is being used, the fragmentation occurring (xfs_bmap
>> is your friend) and the samba config, there's little that can be
>> suggested here.
>
> Very few people configure Samba to do sync writes, but even without
> that I see files written by Samba heavily fragmented when the systems
> operates under intense memory pressure.  The CIFS protocol has very
> small write sizes, and it seems like they do not always arrive in order.
> If the system is under enough memory pressure to write them out already
> before the previous fragment arrive it can cause very fragmented
> extent maps.  Enabling the "strict allocate" option in newer Samba
> version fixes that problem by doing fallocates in larger chunks, which
> from my observation always happen in order.  The downside is that
> current fallocate in XFS is not designed to be used in a fastpath,
> and thus causes performance degradations during the actual write.

Is there anything what i can do in proc/sys/vm configuration to solve the 
fragment write problem?

Thanks,
Janos