Re: [Qemu-devel] [PATCH block-next 0/3] qemu-img check/qcow2: Allow fixing refcounts

[Zhi Yong Wu]

On Thu, May 31, 2012 at 5:26 PM, Stefan Hajnoczi <address@hidden> wrote:
> On Wed, May 30, 2012 at 9:31 AM, Zhi Yong Wu <address@hidden> wrote:
>> On Sat, May 12, 2012 at 12:48 AM, Kevin Wolf <address@hidden> wrote:
>>> A prerequisite for a "QED mode" in qcow2, which doesn't update the refcount
>> Recently some new concepts such as "QED mode" in qcow2 are seen
>> frequencely, can anyone explain what it means? thanks.
>
> qcow2 has more metadata than qed.  More metadata means more write
> operations when allocating new clusters.
>
> In order to overcome this performance issue qcow2 has a metadata
> cache.  But when QEMU is launched with -drive ...,cache=writethrough
> (the default) the metadata cache *must* be in writethrough mode
Why must i be? If the option with -drive ..,cache=writethrough is
specified. it means that host page cache is on while guest disk cache
is off. Since the metadata cache exists in host page cache, not guest,
i think that it is in writeback mode.

> instead of writeback mode.  In other words, every metadata update
> needs to be written to the image file before we complete the guest's
What will mean one guest's wirte request is completed?
> write request.  This means the metadata cache only hides the metadata
> performance issue when -drive ...,cache=direct|writeback are used
> because there we can keep metadata changes buffered in memory until
> the guest flushes the emulated disk write cache.
>
> "QED mode" is a solution for -drive ...,cache=writethrough|directsync.
>  It simply doesn't update refcount metadata in the qcow2 image file
> immediately in exchange for a refcount fixup step that is introduced
Can you say this with more details? Why is this step need only when
image file is opened? After image file is opened, and some guest's
write requests are completed, maybe the refcount fixup step need to be
done once.
> when opening the image file.  It's like doing an fsck operation on a
> file system when mounting it.
>
> Stefan



-- 
Regards,

Zhi Yong Wu