On Thu, Aug 20, 2020 at 15:31:28 +0200, Michal Privoznik wrote: > On 8/20/20 1:57 PM, Peter Krempa wrote: > > On Tue, Jul 07, 2020 at 21:46:31 +0200, Michal Privoznik wrote: > > > When handling sparse stream, a thread is executed. This thread > > > runs a read() or write() loop (depending what API is called; in > > > this case it's virStorageVolDownload() and this the thread run > > > read() loop). The read() is handled in virFDStreamThreadDoRead() > > > which is then data/hole section aware, meaning it uses > > > virFileInData() to detect data and hole sections and sends > > > TYPE_DATA or TYPE_HOLE virStream messages accordingly. > > > > > > However, virFileInData() does not work with block devices. Simply > > > because block devices don't have data and hole sections. But we > > > can use new virFileInDataDetectZeroes() which is block device > > > friendly for that. > > > > > > Partially resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1852528 > > > > > > Signed-off-by: Michal Privoznik <mprivozn@xxxxxxxxxx> > > > --- > > > src/util/virfdstream.c | 15 ++++++++++++--- > > > 1 file changed, 12 insertions(+), 3 deletions(-) > > > > IMO this goes against the semantics of the _SPARSE_STREAM flag. A block > > device by definition is not sparse, so there are no holes to send. > > > > What you've implemented is a way to sparsify a block device, but that > > IMO should not be considered by default when a block device is used. > > If a file is not sparse, the previous code doesn't actually transmit > > holes either. > > > > If you want to achieve sparsification on the source side of the > > transmission, this IMO needs an explicit flag to opt-in and then we > > should sparsify also regular files using the same algorithm. > > > > Fair enough. So how about I'll send v3 where: > > a) in the first patches I make our stream read/write functions handle block > devices for _SPARSE_STREAM without any zero block detection. Only thing that > will happen is that if the source is a sparse regular file and thus the > stream receiver gets a HOLE packet and it is writing the data into a block > device it will have to emulate the hole by writing block of zeroes. However, > if the stream source is a block device then no HOLE shall ever be sent. AFAIK I've R-b'd enough patches to fix this portion and provided that there aren't any merge conflicts you can already commit those. I'm completely fine with that portion as-is. > b) in next patches I'll introduce _DETECT_ZEROES flag (and possibly make it > require _SPARSE_STREAM too) which will handle the case where the stream > source is a block device with zero blocks, at which point it will try to > detect them and be allowed to send HOLE down the stream. On this topic, I agree that it's a sensible approach for the rest of the series and it at least unifies the behaviour. I'm unsure though whether it's worth even doing _DETECT_ZEROES feature at all though. To me it feels that the users are better off using other tools rather than re-implementing yet another thing in libvirt. If possible provide some additional justification here.
