Re: [Qemu-devel] [PATCH] qcow2: Metadata preallocation

[Avi Kivity <avi@redhat.com>]

On 08/17/2009 10:11 AM, Kevin Wolf wrote:
> Avi Kivity schrieb:
>    
>> On 08/14/2009 06:00 PM, Kevin Wolf wrote:
>>      
>>> This introduces a qemu-img create option for qcow2 which allows the metadata to
>>> be preallocated, i.e. clusters are reserved in the refcount table and L1/L2
>>> tables, but no data is written to them. Metadata is quite small, so this
>>> happens in almost no time.
>>>
>>> Especially with qcow2 on virtio this helps to gain a bit of performance during
>>> the initial writes. However, as soon as create a snapshot, we're back to the
>>> normal slow speed, obviously. So this isn't the real fix, but kind of a cheat
>>> while we're still having trouble with qcow2 on virtio.
>>>
>>> Note that the option is disabled by default and needs to be specified
>>> explicitly using qemu-img create -f qcow2 -o preallocation=metadata.
>>>
>>>
>>>        
>> Can't say I'm thrilled with this.  I'd prefer coalescing metadata
>> updates on parallel writes.  I don't object to this though.
>>      
> Even with improved concurrent cluster allocation, you might profit from
> metadata preallocation by having less fragmented qcow2 images which
> avoids splitting up requests. Not sure if this is relevant in practice
> though.
>    

What I meant was that I prefer changes that improve performance 
throughout the lifetime of the image rather than the initial writes, 
especially as there's a space tradeoff.  It is not a strong objection, 
just a mild preference.

> I'm certainly hoping that raw-win32 is doing whatever needs to be done?
> The mentioned FSCTL_SET_SPARSE seems to be there at least.
>    

Ah, I looked at the open code and missed it.

-- 
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