Re: Reflink (cow) copy of busy files

[Gionatan Danti <g.danti@assyoma.it>]

Il 26-02-2018 18:26 Darrick J. Wong ha scritto:
> The way reflink is supposed to work wrt consistency is:
> 
> 1. lock out all new io/fallocate activity on both inodes 
> (iolock/mmaplock)
> 2. wait for all directio to complete
> 3. fsync both files (write all the dirty pagecache to disk)
> 4. lock both inodes (ilock)
> 5. clone each extent atomically
> 6. unlock ilock
> 7. unlock iolock/mmaplock
> 
> So at least in theory the cloned file will match whatever the host saw
> on disk and page cache at the time the reflink call was initiated.
> I say 'in theory' because there could be bugs.

Great! CoW will be a great addition for XFS when it will be considered 
stable.

> Whatever dirty state is in the guest VM stays in that VM, which means
> that if you only cp --reflink on the host, the clone you get will
> reflect the virtual disk state as if you'd kill -9'd the VM, cloned the
> VM disk, and restarted the VM.  Upon restart the log recovers whatever
> metadata made it out of the VM.

Sure, it is what I means for "crash-consistent".

> However, if you tell the guest to freeze the fs before cloning (as Dave
> suggested earlier) the guest will flush all its state to the upper 
> level
> (the host) and the host will push all that out to disk before cloning.
> The snapshot you create should be cleaner because you're effectively
> prepaying the recovery costs by flushing everything before taking the
> snapshot.

True, and this is "application-level consistency" (which requires a 
guest agent and possibly even an application-specific agent)

> Also note that if the host goes down before returning from the syscall,
> the log will continue on with whichever extent was being cloned at the
> time in order to preserve metadata integrity, but the destination file
> will reflect a partial copy.

Thanks for pointing that, and for your extremely clear explanation!


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Danti Gionatan
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