linux-kernel - Re: XFS internal error (memory corruption)

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Date:	Wed, 6 Jul 2011 03:07:56 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	T?r?k Edwin <edwintorok@...il.com>, xfs-masters@....sgi.com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	xfs@....sgi.com
Subject: Re: XFS internal error (memory corruption)

On Wed, Jul 06, 2011 at 02:04:03PM +1000, Dave Chinner wrote:
> > Although is there supposed to be a performance benefit from having
> > a separate log disk with XFS?
> 
> There used to be. Now everyone just uses delayed logging, which is
> far faster and more scalable that even using an external log.

Even with delayed logging external logs are a huge benefit if you
hit the log hard, e.g. for fsync intensive workloads.  E.g. when
using fs_mark in fsync mode it gives speedups over 100% for the
setups I've tested.  You'll see similar speedups for NFS server
loads that are log force heavy as well.

> 
> > IIRC it has a disadvantage that you can't use barriers properly.
> 
> That mostly works now (recent kernels), but you take a hit in
> journal IO waiting synchronously for the data device caches to be
> flushed before writing to the log device.

For metadata-heavy workloads where an external log benefits you most
you generally just want to disable the volatile write cache anyway.

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