Re: [PATCH 2/2] qapi: implementation of the block-dirty-bitmap-dump command

[Kevin Wolf]

Am 20.03.2021 um 10:32 hat Patrik Janoušek geschrieben:
> Currently, dirty bitmaps are for internal use only and there is
> no support for accessing their content from third party-apps.
> This patch implements new command block-dirty-bitmap-dump, which
> returns content of the dirty bitmap encoded in base64. This is
> very useful especially in combination with a drive that uses raw
> format because third-party apps can easily use it to create
> incremental or differential backup.
> 
> Signed-off-by: Patrik Janoušek <pj@patrikjanousek.cz>

People have already pointed you to NBD to get the block dirty status
(and they are right), but I think nobody has explained yet why we
decided against a QMP command to return bitmaps.

The problem is that disk images can be huge, and this means that dirty
bitmaps describing them get pretty large, too. So your new QMP command
ends up reading many megabytes from disk and sending it to the socket.
While it's doing all of this I/O, QEMU is blocked and the guest may
appear to be hanging until it completes.

The client would also have to download the whole bitmap even if it is
only interested in specific blocks. (This may or may not be relevant for
you specific use case.)

With the NBD export, the client can query a specific block range and its
request is processed asynchronously in the background while the guest
and the QMP monitor are still available.

Kevin