On 05/17/2010 03:58 PM, Anthony Liguori wrote:
On 05/17/2010 05:14 AM, Alexander Graf wrote:
Usually the guest can tell the host to flush data to disk. In some
cases we
don't want to flush though, but try to keep everything in cache.
So let's add a new cache value to -drive that allows us to set the cache
policy to most aggressive, disabling flushes. We call this mode
"volatile",
as guest data is not guaranteed to survive host crashes anymore.
This patch also adds a noop function for aio, so we can do nothing in
AIO
fashion.
Signed-off-by: Alexander Graf<ag...@suse.de>
I'd like to see some performance data with at least an ext3 host file
system and an ext4 file system.
My concern is that ext3 exaggerates the cost of fsync() which will
result in diminishing value over time for this feature as people move
to ext4/btrfs.
In fact, btrfs is currently unusable for virt because O_SYNC writes
inflate a guest write to a host write. by a huge factor (50x-100x).
cache=writethrough is 100% unusable, cache=writeback is barely
tolerable. As of 2.6.32, cache=volatile is probably required to get
something resembling reasonable performance on btrfs.
Of course, we expect that btrfs will improve in time, but still it
doesn't seem to be fsync friendly.
--
error compiling committee.c: too many arguments to function
