On 08/17/2010 09:45 AM, Christoph Hellwig wrote:
On Tue, Aug 17, 2010 at 09:39:15AM -0500, Anthony Liguori wrote:
The type of cache we present to the guest only should relate to how
the hypervisor caches the storage. It should be independent of how
data is cached by the disk.
It is.
There can be many levels of caching in a storage hierarchy and each
hierarchy cached independently of the next level.
If the user has a disk with a writeback cache, if we expose a
writethrough cache to the guest, it's not our responsibility to make
sure that we break through the writeback cache on the disk.
The users doesn't know or have to care about the caching. The
users uses O_SYNC/fsync to tell it wants data on disk, and it's the
operating systems job to make that happen. The situation with qemu
is the same - if we tell the guest that we do not have a volatile write
cache that needs explicit management the guest can rely on the fact
that it does not have to do manual cache management.
This is simply unrealistic. O_SYNC might force data to be on a platter
when using a directly attached disk but many NAS's actually do writeback
caching and relying on having an UPS to preserve data integrity.
There's really no way in the general case to ensure that data is
actually on a platter once you've involved a complex storage setup or
you assume FUA
Let me put it another way. If an admin knows the disks on a machine
have battery backed cache, he's likely to leave writeback caching enabled.
We are currently giving the admin two choices with QEMU, either ignore
the fact that the disk is battery backed and do write through caching of
the disk or do writeback caching in the host which expands the disk
cache from something very small and non-volatile (the on-disk cache) to
something very large and volatile (the page cache). To make the page
cache non-volatile, you would need to have an UPS for the hypervisor
with enough power to flush the page cache.
So basically, we're not presenting a model that makes sensible use of
reliable disks.
cache=none does the right thing here but doesn't benefit from the host's
page cache for reads. This is really the missing behavior.
Regards,
Anthony Liguori
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