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SubjectRe: [PATCH][RF C/T/D] Unmapped page cache control - via boot parameter
On Mon, Mar 15, 2010 at 06:43:06PM -0500, Anthony Liguori wrote:
> I knew someone would do this...
>
> This really gets down to your definition of "safe" behaviour. As it
> stands, if you suffer a power outage, it may lead to guest corruption.
>
> While we are correct in advertising a write-cache, write-caches are
> volatile and should a drive lose power, it could lead to data
> corruption. Enterprise disks tend to have battery backed write caches
> to prevent this.
>
> In the set up you're emulating, the host is acting as a giant write
> cache. Should your host fail, you can get data corruption.
>
> cache=writethrough provides a much stronger data guarantee. Even in the
> event of a host failure, data integrity will be preserved.

Actually cache=writeback is as safe as any normal host is with a
volatile disk cache, except that in this case the disk cache is
actually a lot larger. With a properly implemented filesystem this
will never cause corruption. You will lose recent updates after
the last sync/fsync/etc up to the size of the cache, but filesystem
metadata should never be corrupted, and data that has been forced to
disk using fsync/O_SYNC should never be lost either. If it is that's
a bug somewhere in the stack, but in my powerfail testing we never did
so using xfs or ext3/4 after I fixed up the fsync code in the latter
two.



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