Re: JFYI: ext4 bug triggerable by kvm

Avi Kivity
2010-08-17T15:01:41Z

On 08/17/2010 05:54 PM, Anthony Liguori wrote:
>
> 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

That's fine. Memory backed up by a UPS is a disk platter as far as the
user is concerned, if the NAS is reliable.

>
> 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.

In this case, as far as the host is concerned, there is no cache. Data
written is guaranteed to reach the disk eventually even without a
flush. Hopefully the disk advertises itself as not having a volatile cache.

--
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