Re: [Qemu-devel] [PATCH 1/5] RFC: Efficient VM backup for qemu (v1)

[Paolo Bonzini]

Il 21/11/2012 13:37, Kevin Wolf ha scritto:
>>> >> The active part is a bit more tricky. You're putting some code into 
>>> >> block.c to
>>> >> achieve it, which is kind of ugly. 
>> > 
>> > yes. but I tried to keep that small ;-)
> Yup, it's already not too bad. I haven't looked into it in much detail,
> but I'd like to reduce it even a bit more. In particular, the
> backup_info field in the BlockDriverState feels wrong to me. In the long
> term the generic block layer shouldn't know at all what a backup is, and
> baking it into BDS couples it very tightly.

My plan was to have something like bs->job->job_type->{before,after}_write.

   int coroutine_fn (*before_write)(BlockDriverState *bs,
        int64_t sector_num, int nb_sectors, QEMUIOVector *qiov,
        void **cookie);
   int coroutine_fn (*after_write)(BlockDriverState *bs,
        int64_t sector_num, int nb_sectors, QEMUIOVector *qiov,
        void *cookie);


The before_write could optionally return a "cookie" that is passed back
to the after_write callback.

Actually this was plan B, as a poor-man implementation of the filter
infrastructure.  Plan A was that the block filters would materialize
suddenly in someone's git tree.

Anyway, it should be very easy to convert Dietmar's code to something
like that, and the active mirror could use it as well.

>> > AFAIK qcow2 file cannot store data out of order. In general, an backup fd 
>> > is not seekable, 
>> > and we only want to do sequential writes. Image format always requires 
>> > seekable fds?
> Ah, this is what you mean by "out of order". Just out of curiosity, what
> are these non-seekable backup fds usually?

Perhaps I've been reading the SCSI standards too much lately, but tapes
come to mind. :)

Paolo