Re: [Qemu-block] KVM Forum block no[td]es

[Max Reitz]

On 12.11.18 00:36, Nir Soffer wrote:
> On Mon, Nov 12, 2018 at 12:25 AM Max Reitz <address@hidden
> <mailto:address@hidden>> wrote:
> 
>     This is what I’ve taken from two or three BoF-like get-togethers on
>     blocky things.  Amendments are more than welcome, of course.
> 
> ... 
> 
>     Bitmaps
> 
>     =======
> 
>     (Got this section from sneaking into a BoF I wasn’t invited to.  Oh
>     well.  Won’t hurt to include them here.)
> 
>     Currently, when dirty bitmaps are loaded, all IN_USE bitmaps are just
>     not loaded at all and completely ignored.  That isn’t correct, though,
>     they should either still be loaded (and automatically treated and
>     written back as fully dirty), or at least qemu-img check should
>     “repair” them (i.e. fully dirtying them).
> 
> 
> I'm not sure making bitmaps dirty is better.
> 
> When bitmap is marked IN_USE, it means that we cannot use it for
> backup. Deleting the bitmap or making it as bad so it cannot be used
> for the next backup is the same. Making the bitmap as dirty will full
> the management layer that everything was fine when the next backup
> includes the entire disk. It is better to cause the next backup to fail
> in a verbose way, since the backup software can recover by doing
> a full backup.

But making the dirty bitmap fully dirty will automatically enforce a
full backup.

>     Sometimes qemu (running in a mode as bare as possible) is better than
>     using qemu-img convert, for instance.  It gives you more control
>     (through QMP; you get e.g. better progress reporting), you get all of
>     the mirror optimizations (we do have optimizations for convert, too,
>     but whether it’s any good to write the same (or different?)
>     optimizations twice is another question), and you get a common
>     interface for everything (online and offline).
>     Note that besides a bare qemu we’ve also always wanted to convert as
>     many qemu-img operations into frontends for block jobs as possible.
>     We have only done this for commit, however, even though convert looked
>     like basically the ideal target.  It was just too hard with too little
>     apparent gain, like always (and convert supports additional features
>     like concatenation which we don’t have in the runtime block layer
>     yet).
> 
> 
> I'm not sure it is better to run qemu and use qemu-img as thin wrapper
> over qmp.
> 
> For example, management system may ascociate qemu-img
> with a sanlock lease, and sanlock may try to terminate qemu-img when
> a lease is invalidated. In this case sanlock will succeed while qemu is till
> accessing storage it should not access.
> ...

The point was not to run two processes, but to link the necessary bits
of qemu into qemu-img and then use them inside the single qemu-img
process itself.  As hinted at above, we've been doing this for qemu-img
commit for quite some time; that command actually runs a block job under
the hood (inside of qemu-img).

Max

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