Re: [Qemu-devel] [RFC] Propose the Fast Virtual Disk (FVD) image format that outperforms QCOW2 by 249%

[Stefan Hajnoczi]

On Wed, Jan 19, 2011 at 1:12 AM, Jamie Lokier <address@hidden> wrote:
> Chunqiang Tang wrote:
>> Moreover, using a host file system not only adds overhead, but
>> also introduces data integrity issues. Specifically, if I/Os uses O_DSYNC,
>> it may be too slow. If I/Os use O_DIRECT, it cannot guarantee data
>> integrity in the event of a host crash. See
>> http://lwn.net/Articles/348739/ .
>
> You have the same issue with O_DIRECT when using a raw disk device
> too.  That is, O_DIRECT on a raw device does not guarantee integrity
> in the event of a host crash either, for mostly the same reasons.

QEMU has semantics that use O_DIRECT safely; there is no issue here.
When a drive is added with cache=none, QEMU not only uses O_DIRECT but
also advertises an enabled write cache to the guest.

The guest *must* flush the cache when it wants to ensure data is
stable.  In the event of a host crash, all, some, or none of the I/O
since the last flush may have made it to disk.  Each of these
possibilities is fair game since the guest may only depend on writes
being on disk if they completed and a successful flush was issued
afterwards.

Stefan