Re: [Qemu-devel] [RFC PATCH] drive-backup 'stream' mode

[Fam Zheng]

On Fri, 10/11 11:18, Wolfgang Richter wrote:
> Idea: Introduce a mode for drive-backup that duplicates writes to
> another target, not CoW.  It is useful for introspecting (my use
> case), and for keeping a remote block device in sync with writes
> (helps with migration or backup).
> 
> 
> 
> Issue with current modes:  All of the current modes are well-designed
> to support point-in-time snapshots, but none of them handle keeping
> another drive up-to-date as new writes continuously occur.  The 'None'
> mode documentation is a bit ambiguous in this regard, but what it
> actually implements is a very low overhead CoW snapshot.
> 
> 
> 
> Patch: Fixes ambiguity in the 'None' mode documentation, introduces a
> new mode 'stream' which duplicates writes without reading any data
> from the original disk.
> 
> I put the logic for copying the write into a new coroutine called
> 'backup_do_stream' as it needs almost nothing from the original
> 'backup_do_cow' function (no bit map, no reads from a block device,
> etc.).  The other major change is that tracked requests also contain a
> handle to the QIOV involved in the write (and it is passed along).
> 
> This is based off of v1.6.0 code.
> 

While mirroring write is a good idea, doing it with drive-backup is probably
not. The function of this command is to 'backup' the image with existing data,
instead of new data. With your 'stream' mode, this semantic is changed.

IMO this feature is best implemented as a block filter, which is currently
being discussed and not ready yet. A second option may be doing with another
command (e.g. block-mirror, or a new one?)

Thanks,
Fam