[PATCH 0/9] Allow sparse streams for block devices

[Michal Privoznik]

On 7/3/20 1:34 PM, Daniel P. Berrangé wrote:
> On Fri, Jul 03, 2020 at 12:28:41PM +0200, Michal Privoznik wrote:
>> The way our sparse streams are implemented is:
>>
>> 1) user facing APIs (virStreamSparseRecvAll() and virStreamSparseSendAll()) take
>>     callbacks as arguments. These callbacks read/write data or determine if there
>>     is a hole in the underlying file and big it is.
>>
>> 2) libvirtd has something similar - virFDStream, except here the functions for
>>     read/write of data and hole handling are called directly.
>>
>> Sparse streams were originally implemented for regular files only => both ends
>> of stream has to be regular files. This limitation exists because the callbacks
>> from 1) (implemented in virsh for vol-download/vol-upload commands) and also
>> from 2) (which is basically the same code) uses lseek(..., SEEK_DATA) and/or
>> lseek(..., SEEK_HOLE) to get map of allocated file blocks. They also take a
>> shortcut (valid for regular files) - when one side of the stream is asked to
>> create a hole it merely lseek() + ftruncate(). For regular files this creates a
>> hole and later, when somebody reads it all they get is zeroes.
>>
>> Neither of these two approaches work for block devices. Block devices have no
>> notion of data/hole sections [1], nor can they be truncated. What we can do
>> instead is read data from the block device and check if its full of zeroes. And
>> for "creating a hole" we just write zeroes of requested size.
>>
>> There is a follow up patch that I am working on: this implementation I'm
>> posting here has one disadvantage: after some blocks are read from the
>> block device and they are found to contain data, the whole buffer is
>> freed only to be read again. For instance, when uploading volume,
>> virshStreamInData() is called at the beginning to check if the file
>> containing data to upload doesn't start with a hole. If the file is a
>> block device, then virFileInDataDetectZeroes() is called which reads
>> 1MiB from it, finds (say) data and throws the buffer away. Then
>> virshStreamSource() is called, which reads the 1MiB buffer again.
>> The patch is still under development though.
> 
> Was there a particular user/app feature request for this support ?

Yes, see BZ linked in 9/9.

https://bugzilla.redhat.com/show_bug.cgi?id=1852528

Apparently, VDSM uses streams to copy volumes around.

> 
> I'm wondering about the likely use case, because if I was starting
> over from scratch I'd never implement stream support for storage
> volumes. Instead I would add APIs for starting/stopping a qemu-nbd
> server attached to the volume. Probably don't even need a start/
> stop pair, could just run in single client mode where we pass back
> an opened client FD, and have qemu-nbd exit when this is closed.
> 
> Depending on the user requesting sparse support for blockdevs it
> may still make sense to provide them an NBD solution, especially
> if they're likely to have followup feature requests already handled
> by NBD.

Agreed, streams should have been for console and screenshot only. They 
are strictly worse for handling large files than scp/nbd/rsync/... 
because it all has to go through our event loop. And client even loop.
On the other hand, they are multiplexed within virCommand which is an 
advantage that neither of the aforementioned tools have (no need to open 
a special port then).

Michal