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

[Anthony Liguori]

Ryan Harper wrote:
> * Anthony Liguori <address@hidden> [2008-10-09 12:00]:
>   
> > 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.
> >
> >     

With 16k writes I think we hit a pathological case with the particular 
storage backend we're using since it has many disks and the volume is 
striped.  Also the results a bit different when going through a file 
system verses a LVM partition (the later being the first data set).  
Presumably, this is because even with no flags, writes happen 
synchronously to a LVM partition.

Also, cache=off seems to do pretty terribly when operating on an ext3 
file.  I suspect this has to do with how ext3 implements O_DIRECT.

However, the data demonstrates pretty nicely that O_DSYNC gives you 
native write speed, but accelerated read speed which I think we agree is 
the desirable behavior.  cache=off never seems to outperform cache=wt 
which is another good argument for it being the default over cache=off.

Regards,

Anthony Liguori