Christoph Hellwig wrote:
On Mon, May 11, 2009 at 09:51:40AM -0500, Anthony Liguori wrote:
What typically triggers a flush operation?
fsync.
I would assume an fsync would, but would a flush happen after every
O_DIRECT write?
Right now it doesn't, but it probably should.
So then with cache=writeback, fsync behaves itself but O_DIRECT writes
do not.
This seems like a really undesirable combination of behavior from a
guest integrity point of view. It makes me wonder if it's really
useful. I think that any serious user would have to continue using
cache=writethrough. Is there a path that would ever allow someone who
cares about their data to use cache=writeback instead of cache=writethrough?
If the backend implementation of T_FLUSH is fsync, I would think that
this would result in rather poor performance for O_DIRECT operations in
the guest.
Right now it's fsync. By the time I'll submit the backend change it
will still be fsync, but at least called from the posix-aio-compat
thread pool.
fsync is pretty crappy on ext3 default configs. I'm concerned that this
could be considered a DoS by a malicious guest. If it sat in a T_FLUSH
loop, it would potentially bring your system to a crawl, no?
Regards,
Anthony Liguori
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