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

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.

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.