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Re: journal on an ssd



On Sep 12, 2008  10:17 +0200, Keld J�rn Simonsen wrote:
> On Thu, Sep 11, 2008 at 03:07:01PM -0600, Andreas Dilger wrote:
> > On Sep 11, 2008  07:43 +0200, Tobias Oetiker wrote:
> > > Because if it is catastrophic, then it basically means that the
> > > journal has to reside on a device that is as secure as to rest of
> > > the data, meaning that if the data is on RAID6 then the journal
> > > should be on RAID6 too.
> > 
> > No, because RAID6 is terribly sucky for performance.  If you need this
> > kind of reliability triple-mirrored RAID 1 would be better.  Much less
> > CPU overhead, and no extra IO.
> 
> RAID6 performs nicely for reads, but has quite bad performance for some
> writes (non-sequential). Raid6 is actually surprisingly fast for
> sequential reads.

The journal is NEVER read during normal operation, only once during
journal recovery after a crash.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.


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