Re: JFYI: ext4 bug triggerable by kvm

Anthony Liguori
2010-08-17T14:39:20Z

On 08/17/2010 09:28 AM, Christoph Hellwig wrote:
> On Tue, Aug 17, 2010 at 09:20:37AM -0500, Anthony Liguori wrote:
>
>> On 08/17/2010 08:07 AM, Christoph Hellwig wrote:
>>
>>>> The point is that we don't want to flush the disk write cache. The
>>>> intention of writethrough is not to make the disk cache writethrough
>>>> but to treat the host's cache as writethrough.
>>>>
>>> We need to make sure data is not in the disk write cache if want to
>>> provide data integrity.
>>>
>> When the guest explicitly flushes the emulated disk's write cache.
>> Not on every single write completion.
>>
> That depends on the cache= mode. For cache=none and cache=writeback
> we present a write-back cache to the guest, and the guest does explicit
> cache flushes. For cache=writethrough we present a writethrough cache
> to the guest, and we need to make sure data actually has hit the disk
> before returning I/O completion to the guest.
>

Why?

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.

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.

Regards,

Anthony Liguori