Re: [PATCH v2 13/17] virfdstream: Allow sparse stream vol-download

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(a)redhat.com&gt; >>> --- >>> 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.