Re: [PATCH, RFC] virtio_blk: add cache flush command

[Anthony Liguori <anthony@codemonkey.ws>]

Avi Kivity wrote:
> Anthony Liguori wrote:
>
>>
>> Right now, it's fairly easy to understand.  cache=none and 
>> cache=writethrough guarantee that all write operations that the guest 
>> thinks have completed are completed.  cache=writeback provides no 
>> such guarantee.
>
> cache=none is partially broken as well, since O_DIRECT writes might 
> hit an un-battery-packed write cache.  I think cache=writeback will 
> send the necessary flushes, if the disk and the underlying filesystem 
> support them.

Sure, but this likely doesn't upset people that much since O_DIRECT has 
always had this behavior.  Using non-battery backed disks with writeback 
enabled introduces a larger set of possible data integrity issues.  I 
think this case is acceptable to ignore because it's a straight forward 
policy.

>> cache=writeback+fsync would guarantee that only operations that 
>> include a T_FLUSH are present on disk which currently includes fsyncs 
>> but does not include O_DIRECT writes.  I guess whether O_SYNC does a 
>> T_FLUSH also has to be determined.
>>
>> It seems too complicated to me.  If we could provide a mode where 
>> cache=writeback provided as strong a guarantee as cache=writethrough, 
>> then that would be quite interesting.
>
> It don't think we realistically can.

Maybe two fds?  One open in O_SYNC and one not.  Is such a thing sane?

>>>> (Or maybe ext3 actually is stupid enough to flush the whole fs even 
>>>> for
>>>> that case
>>>
>>> Sigh.
>>
>> I'm also worried about ext3 here.
>
> I'm just waiting for btrfs.

Even ext4 is saner but we'll get lots of bug reports while ext3 remains 
common.

Regards,

Anthony Liguori