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

[Peter Krempa]

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 at 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.