Re: [Qemu-devel] [PATCH v5 03/11] block: add basic backup support to block driver

[Fam Zheng <famz@redhat.com>]

On Thu, 06/13 14:07, Wenchao Xia wrote:
> 于 2013-6-13 14:03, Wenchao Xia 写道:
> >于 2013-6-7 15:18, Stefan Hajnoczi 写道:
> >>On Thu, Jun 06, 2013 at 04:56:49PM +0800, Fam Zheng wrote:
> >>>On Thu, 06/06 10:05, Stefan Hajnoczi wrote:
> >>>>On Thu, Jun 06, 2013 at 11:56:18AM +0800, Fam Zheng wrote:
> >>>>>On Thu, 05/30 14:34, Stefan Hajnoczi wrote:
> >>>>>>+
> >>>>>>+static int coroutine_fn backup_before_write_notify(
> >>>>>>+        NotifierWithReturn *notifier,
> >>>>>>+        void *opaque)
> >>>>>>+{
> >>>>>>+    BdrvTrackedRequest *req = opaque;
> >>>>>>+
> >>>>>>+    return backup_do_cow(req->bs, req->sector_num,
> >>>>>>req->nb_sectors, NULL);
> >>>>>>+}
> >>>>>
> >>>>>I'm wondering if we can see the logic here with a backing hd
> >>>>>relationship?  req->bs is a backing file of job->target, but guest is
> >>>>>going to write to it, so we need to COW down the data to job->target
> >>>>>before overwritting (i.e.  cluster is not allocated in child).
> >>>>>
> >>>>>I think if we do this in block layer, there's not much necessity for a
> >>>>>before-write notifier here (although it may be useful for other
> >>>>>cases):
> >>>>>
> >>>>>     in bdrv_write:
> >>>>>     for child in req->bs->open_children
> >>>>>         if not child->is_allocated(req->sectors)
> >>>>>             do COW to child
> >>>>>
> >>>>>The advantage of this is that we won't need to start block-backup
> >>>>>job in
> >>>>>sync mode "none" to do point-in-time snapshot (image fleecing), and we
> >>>>>get writable snapshot (possibility to open backing file writable and
> >>>>>write to it safely) as a by-product.
> >>>>>
> >>>>>But we will need to keep track of parent<->child of block states,
> >>>>>and we
> >>>>>still need to take care of overlapping writing between block job and
> >>>>>guest request.
> >>>>
> >>>>There's one catch here: bs->target may not support backing files, it
> >>>>can
> >>>>be a raw file, for example.  We'll only use backing files for
> >>>>point-in-time snapshots but other use cases might not.  raw doesn't
> >>>>really implement is_allocated(), so the whole concept would have to
> >>>>change a little:
> >>>
> >>>Another use case may be parent modification. Suppose we have
> >>>
> >>>                     ,--- child1.qcow2
> >>>     parent.qcow2  <
> >>>                     `--- child2.qcow2
> >>>
> >>>We can use parent.qcow2 as block device in QEMU without breaking
> >>>child1.qcow2 or child2.qcow2 by telling QEMU who its children are:
> >>>
> >>>   $QEMU -drive file=parent.qcow2,children=child1.qcow2:child2.qcow2
> >>>
> >>>Then we open the three images and setup parent_bs->open_children, the
> >>>children are protected from being corrupted.
> >>>
> >>>>
> >>>>bs->open_children becomes independent of backing files - any
> >>>>BlockDriverState can be added to this list.  ->is_allocated() basically
> >>>>becomes the bitmap that we keep in the block job.
> >>>
> >>>Yes. But it is possible to keep a bitmap for raw (and those don't
> >>>implement is_allocated()) in block layer too, or in overlay: could
> >>>add-cow by Dongxu Wang help here?
> >>
> >>Yes absolutely.
> >>
> >>Stefan
> >>
> >   One advantage of external backup, or backing up chain, is that it
> >holds 'Delta' data only and is small enough. If it is changed toward a
> >'full' data writable snapshot, it become bigger. With backup chain
> >qemu-img can restore/clone a writable and usable one, So I don't
> >think adding that in qemu emulator helps much, and it will make things
> >more complicit.... user won't care who is doing the job, qemu or
> >qemu-img.
> >
>   I mean that "get writable snapshot (possibility to open backing file
> writable and write to it safely) as a by-product." in this series, is
> not very valuable.
> 

I'm not selling writable snapshot, my point was just that semantic of
block-backup, getting a point-in-time snapshot, inherently works like a
backing chain but writting to parent (guest drive) will not break its
children (our thin PIT snapshot). If we see it this way, COW is not so
specific to a block job like block-backup, it can be generic in the
backing chain logic.

Though, the value in a writable snapshot is that we can actually
_modify_ a backing image in place, rather than forking the chain to
write to the new child. This is not supported with qemu or qemu-img now,
once you create a child with the image as backing file, you mustn't
modify it.

-- 
Fam