Re: JFYI: ext4 bug triggerable by kvm

Christoph Hellwig
2010-08-17T14:28:10Z

On Tue, Aug 17, 2010 at 09:20:37AM -0500, Anthony Liguori wrote:
> On 08/17/2010 08:07 AM, Christoph Hellwig wrote:
> >>The point is that we don't want to flush the disk write cache. The
> >>intention of writethrough is not to make the disk cache writethrough
> >>but to treat the host's cache as writethrough.
> >
> >We need to make sure data is not in the disk write cache if want to
> >provide data integrity.
>
> When the guest explicitly flushes the emulated disk's write cache.
> Not on every single write completion.

That depends on the cache= mode. For cache=none and cache=writeback
we present a write-back cache to the guest, and the guest does explicit
cache flushes. For cache=writethrough we present a writethrough cache
to the guest, and we need to make sure data actually has hit the disk
before returning I/O completion to the guest.

> > It has nothing to do with the qemu caching
> >mode - for data=writeback or none it's commited as part of the fdatasync
> >call, and for data=writethrough it's commited as part of the O_SYNC
> >write. Note that both these path end up calling the filesystems ->fsync
> >method which is what's require to make writes stable. That's exactly
> >what is missing out in sync_file_range, and that's why that API is not
> >useful at all for data integrity operations.
>
> For normal writes from a guest, we don't need to follow the write
> with an fsync(). We should only need to issue an fsync() given an
> explicit flush from the guest.

Define normal writes. For cache=none and cache=writeback we don't
have to, and instead do explicit calls to fsync()/fdatasync() calls
when a we a cache flush from the guest. For data=writethrough we
guarantee data has made it to disk, and we implement this using
O_DSYNC/O_SYNC when opening the file. That tells the operating system
to not return until data has hit the disk. For Linux this is
internally implement using a range-fsync/fdatasync after the actual
write.

> fsync() being slow is orthogonal to my point. I don't see why we
> need to do an fsync() on *every* write. It should only be necessary
> when a guest injects an actual barrier.

See above.