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@redhat.com> --- 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.