Re: [Qemu-devel] iSCSI support for QEMU

[Christoph Hellwig]

On Thu, Apr 21, 2011 at 06:43:10PM +1000, Ronnie Sahlberg wrote:
> Some basic tests thatve been performed show it to be significantly faster
> than an out-of-the-box open-iscsi mounted LUN being accessed by default
> QEMU i/o options.

Which isn't a useful comparism.  qemu's default is the braindead
cache=writethrough behaviour, which forces writes out to disk, and bloats the
pagecache, while your mail sounds you silently always implement O_DIRECT-like
semantics.

Also the implementation has data integrity issues due this.  It does
not actually force writes out to disk in the default cache mode, despite
qemu claiming to have WCE=0 in the default mode, i.e. you never flush
the cache.  You'll need set the FUA bits on all writes if cache=writethrough
is used, with a fallback to a real cache flush in case the target doesn't
support the FUA bit.