Re: [Qemu-devel] iSCSI support for QEMU

[Christoph Hellwig <hch@lst.de>]

> In my patch, there are NO data integrity issues.
> Data is sent out on the wire immediately as the guest issues the write.
> Once the guest issues a flush call, the flush call will not terminate
> until the SYNCCACHE10 task has completed.

No guest will even issue a cache flush, as we claim to be WCE=0 by default.
Now if you target has WCE=1 it will cache data internally, and your
iscsi initiator will never flush it out to disk.

We only claim WCE=1 to the guest if cache=writeback or cache=none are
set.  So ignoring the issue of having a cache on the initiator side
you must implement stable writes for the default cache=writethrough
behaviour by either seeting the FUA bit on your writes, or doing
a cache flush after every write in case the target does not support FUA.