Re: [Qemu-devel] [RFC] Disk integrity in QEMU

[Gerd Hoffmann]

  Hi,

> Read performance should be unaffected by using O_DSYNC.  O_DIRECT will
> significantly reduce read performance.  I think we should use O_DSYNC by
> default and I have sent out a patch that contains that.  We will follow
> up with benchmarks to demonstrate this.

So O_SYNC on/off is pretty much equivalent to disk write caching being
on/off, right?  So we could make that guest-controlled, i.e. toggeling
write caching in the guest (using hdparm) toggles O_SYNC in qemu?  This
together with disk-flush command support (mapping to fsync on the host)
should allow guests to go into barrier mode for better write performance
without loosing data integrity.

cheers,
  Gerd