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

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.