Ceph » rbd

Note, write serialization is not the only difference between writing to a file and to stdout. Another difference is that when writing to file the export effectively uses lseek to skip image regions without data, while in the case of stdout zeroes are written.

When writing to a device, we can't just lseek zero regions. We need either to write zeros or use low level methods like BLKDISCARD/BLKSECDISCARD/BLKZEROOU ioctl.

This looks like doable but at this moment I am not sure if we need to bloat rbd tool for this specific optimization case.

Ruben, could you please describe your use case(s)? It could help to understand how useful this feature would be.

Couldn't you just run "rbd export <image-spec> - | dd of=/dev/someblockdevice" and achieve the desired outcome?

Mykola Golub wrote:

Note, write serialization is not the only difference between writing to a file and to stdout. Another difference is that when writing to file the export effectively uses lseek to skip image regions without data, while in the case of stdout zeroes are written.

When writing to a device, we can't just lseek zero regions. We need either to write zeros or use low level methods like BLKDISCARD/BLKSECDISCARD/BLKZEROOU ioctl.

This looks like doable but at this moment I am not sure if we need to bloat rbd tool for this specific optimization case.

Ruben, could you please describe your use case(s)? It could help to understand how useful this feature would be.

We run vms on local storage, and store (lvm-based) snapshots of these vms in Ceph using rbd import/export.

Jason Dillaman wrote:

Couldn't you just run "rbd export <image-spec> - | dd of=/dev/someblockdevice" and achieve the desired outcome?

Yes, that's we do now, it's still quite a bit slower though.