Re: [Qemu-devel] Is cache=writeback safe yet?

[Virtbie <virtbie@shiftmail.org>]

On 02/20/12 16:06, Anthony Liguori wrote:
> On 02/20/2012 08:18 AM, Virtbie wrote:
>> Dear qemuers,
>> thanks for your exellent software.
>>
>> I would like to use cache=writeback, but I still can't understand if 
>> this is
>> safe or not in case of power loss.
>
> "Safe" is too simplistic of a view.  The documentation in 
> qemu-options.hx probably needs to be revisited.
>
> cache=writeback emulates a large disk cache much the same as every 
> modern hard drive has a builtin cache.
>
> The only real difference is that the host cache is very, very large.  
> Some modern file systems did not take the necessary steps to ensure 
> consistency when a volatile write cache is present (ext3 up until very 
> recently when barrier=1 became default).
>
> In practice, this didn't create a huge issue because disk write caches 
> are flushed often.  Ted T'so has written a lot about the practical 
> bits here.
>
> But since the host cache is very large, and may not be flushed for 
> many minutes after the initial write, this can exacerbate the problem.
>
> In short, if you're using a recent kernel with ext3 or ext4, 
> cache=writeback is absolutely safe.  If you're using an older version 
> of ext3, cache=writeback is still safe but ext3 itself isn't.  
> cache=writeback can make the situation worse.
>
> cache=writethrough presents a non-volatile cache which makes even 
> older ext3 filesystems safe.
>
> Regards,
>
> Anthony Liguori


Great explanation Anthony,

may I still ask:

1)
Is WCE + volatile flag exposed to the guest, by all three virtual devices:
- virtio
- scsi
- ide
?
(if not, I still don't understand how this works)


2) Is there a minimum guest kernel and a minimum viostor Windows driver 
version, to see such WCE+volatile flag in a virtio disk, so that the 
guest OS can actually see the cache?


Because I can't seem to find such flag for a virtio disk on a linux 
2.6.38 guest and that seems serious to me.
Is it visible somewhere in /sys hierarchy? Or is 2.6.38 too old?


Thank you
Vb.