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

[Chunqiang Tang]

> >> 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.

Thank both of you for the explanation, which is very helpful to me. With 
FVD's capability of eliminating the host file system and storing the image 
on a logical volume, then perhaps we can always use O_DSYNC, because there 
is little (or no?) LVM metadata that needs a flush on every write and 
hence O_DSYNC  does not add overhead? I am not certain on this, and need 
help for confirmation. If this is true, the guest does not need to flush 
the cache.