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

[Daniel P. Berrangé]

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 ?

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.


NB, this is not an objection to your series here. Since you've already
done the work and it is really just giving closer parity between block
and file volumes.

Regards,
Daniel
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