Re: [dm-devel] [PATCH 26/30] ext4: do not send discards as barriers

[Boaz Harrosh <bharrosh panasas com>]

On 08/31/2010 01:13 PM, Tejun Heo wrote:
> Hello,
> 
> On 08/31/2010 12:07 PM, Boaz Harrosh wrote:
>> I don't know all the specifics of the virtio driver and the KVM backend but
>> don't the KVM target io is eventually directed to a local file or device?
>> If so the scsi device has disappeard but the bulk of the data is in host cache
>> at the backstore (file or bdev). Once all files are closed the data is synced
>> to disk.
>>
>> Is it not the same as Ric's problem of disconnecting the sata cable but
>> not dropping power to the drive. The main of the cache is still intact.
> 
> There are two layers of caching there.
> 
>  drive cache - host page cache - guest
> 
> When guest issues FLUSH, qemu will translate it into fdatasync which
> will flush the host page cache followed by FLUSH to the drive which
> will flush the drive cache to the media.  If you delete the host disk
> device, it will be detached w/o host page cache flushed.  So, although
> it's not complete, it will lose good part of cache.  With out write
> out timeout increased and/or with laptop mode enabled, it will
> probably lose most of cache.
> 

Ha, ok you meant that device. So if you have a dedicated physical device
for backstore that would be a very nice scriptable way.

Thanks, that's a much better automated test than pulling drives out of
sockets.

> Thanks.
> 

Boaz