On 03/16/2010 12:26 PM, Christoph Hellwig wrote:
Avi,
cache=writeback can be faster than cache=none for the same reasons
a disk cache speeds up access. As long as the I/O mix contains more
asynchronous then synchronous writes it allows the host to do much
more reordering, only limited by the cache size (which can be quite
huge when using the host pagecache) and the amount of cache flushes
coming from the host. If you have a fsync heavy workload or metadata
operation with a filesystem like the current XFS you will get lots
of cache flushes that make the use of the additional cache limits.
Are you talking about direct volume access or qcow2?
For direct volume access, I still don't get it. The number of barriers
issues by the host must equal (or exceed, but that's pointless) the
number of barriers issued by the guest. cache=writeback allows the host
to reorder writes, but so does cache=none. Where does the difference
come from?
Put it another way. In an unvirtualized environment, if you implement a
write cache in a storage driver (not device), and sync it on a barrier
request, would you expect to see a performance improvement?
If you don't have a of lot of cache flushes, e.g. due to dumb
applications that do not issue fsync, or even run ext3 in it's default
mode never issues cache flushes the benefit will be enormous, but the
data loss and possible corruption will be enormous.
Shouldn't the host never issue cache flushes in this case? (for direct
volume access; qcow2 still needs flushes for metadata integrity).
But even for something like btrfs that does provide data integrity
but issues cache flushes fairly effeciently data=writeback may
provide a quite nice speedup, especially if using multiple guest
accessing the same spindle(s).
But I wouldn't be surprised if IBM's exteme differences are indeed due
to the extremly unsafe ext3 default behaviour.
--
error compiling committee.c: too many arguments to function
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