Re: JFYI: ext4 bug triggerable by kvm

[Avi Kivity <avi@redhat.com>]

  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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