Re: XFS internal error (memory corruption)

Christoph Hellwig
2011-07-06T07:08:05Z

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.