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

[Michal Privoznik]

On 8/20/20 3:42 PM, Peter Krempa wrote:
> 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 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.
>>>
>>
>> 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.

Almost :-)
For instance this very patch uses virFileInDataDetectZeroes() to detect 
zero blocks on block devices. It needs to be changed to always assume 
data section and some length. The same applies to the next patch 14/17.
But the diff is trivial:

iff --git c/src/util/virfdstream.c w/src/util/virfdstream.c
index 9968cdc623..39514ef555 100644
--- c/src/util/virfdstream.c
+++ w/src/util/virfdstream.c
@@ -440,8 +440,15 @@ virFDStreamThreadDoRead(virFDStreamDataPtr fdst,

      if (sparse && *dataLen == 0) {
          if (isBlock) {
-            if (virFileInDataDetectZeroes(fdin, &inData, &sectionLen) < 0)
-                return -1;
+            /* Block devices are always in data section by definition. The
+             * @sectionLen is slightly more tricky. While we could try 
and get
+             * how much bytes is there left until EOF, we can pretend 
there is
+             * always X bytes left and let the saferead() below hit EOF 
(which
+             * is then handled gracefully anyway). Worst case scenario, 
this
+             * branch is called more than once.
+             * X was chosen to be 1MiB but it has ho special meaning. */
+            inData = 1;
+            sectionLen = 1 * 1024 * 1024;

And the same for virsh case. Do you want me to resend those two patches?


> 
>> 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.

Alright. Fair enough I guess.

> 
> If possible provide some additional justification here.
> 

It was discussed in the bz 
https://bugzilla.redhat.com/show_bug.cgi?id=1852528
VDSM is doing a thin provisioning and as a part of that they are copying 
files onto block devices. But for that zero detection shouldn't be needed.

Michal