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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

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      Last update: 2010-03-16 01:45    [W:0.028 / U:15.484 seconds]
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