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