[libvirt-users] Shouldn't vol-upload / virStorageVolUpload() be doing some format conversion?

[Eric Blake]

On 04/12/2013 12:06 PM, Guido Winkelmann wrote:
> Hi,
> 
> I just tried using vol-update to copy an image file to a storage pool, and I 
> noticed that, at least in my configuration (qemu backend, storage pool of type 
> "dir"), this command does not appear to be doing any kind of format 
> conversion. It simply copied the source file as-is over the file in which the 
> target volume was stored with no checks whether that even makes sense.
> 
> The help text to this command says it supports offset and length, which - 
> kinda, sorta - implies that I can write a block of bytes at offset n to a 
> volume, attach that volume to a domain, and then the OS in the guest domain 
> will find exactly the bytes I just wrote at exactly offset n in its newly 
> attached harddisk/CDROM/whatever. Instead, trying to this with anything but 
> raw images would probably just break the image file.
> 
> AFAICS, in its current form, this command and this API-call are only useful in 
> exactly two scenarios:
> 
> - Both the source and target image are in raw format
> 
> - Source and target image are in the same non-raw format (like qcow2 or vmdk) 
> AND you are copying the entire volume at once.

Yeah, and I'd LOVE to enhance that by use of flags, so that we can allow
raw->formatted and formatted->raw conversion by adding a couple of flags
(that is, are we uploading the offset in relation to the HOST's view of
the file, or in relation to the GUEST's view of the file).  Going from
format A to format B would be a matter of hooking up a volumeDownload
from one format to a volumeUpload in the other format, so it might also
make sense to enhance the virStream API to make it easier to pipeline
two streams end-to-end.  But someone has to write the patches.

> 
> In all other cases - source and target are in different formats, source and 
> target are in non-raw and you are trying to write only parts of a volume - 
> this can only lead to a broken image file.

Usually, you want to be copying the offsets in relation to the guest's
view of the storage; it is very seldom that you would want to copy the
host's view of a non-raw file (except, as you note, when copying the
entire file including metadata).  The fact that the existing command
treats offsets relative to the host's view is annoying; but for backward
compatibility, I think we're stuck with it.

> 
> As far as I see it, at the layer of abstraction libvirt offers, "volume" 
> should not be synonymous with "the file the file the volume is stored in on 
> disk", but rather with "the volume as a guest machine will get to see it".

There's certainly lots of room for improvement in the virStorageVol* API.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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