Re: RBD Kernel: write_same_max_bytes and SCSI XCOPY

[Mike Christie <mchristi@redhat.com>]

On 12/09/2015 01:08 AM, Timofey Titovets wrote:
> Hi list,
> As i found write_same_max_bytes are equal to zero
> if i understand correctly, it's means what RBD not support offloading
> copy operations
> 

Not yet.

I did patches to support commands like write same and compare and write
in this patchset where I made a lio rbd backend driver:

http://www.spinics.net/lists/target-devel/msg10330.html

but they were not accepted upstream.

I am currently reworking the patches to be acceptable. Some distros did
pick them up though so you might see them in there.


> Is this possible to add this feature, as example for LIO with SCSI XCOPY?


The linux kernel does yet not have xcopy support like how it supports
write same or discard. There are patches, but it has not been merged
upstream.

However, LIO does support this command by what I guess you could call
emulating it where if the underlying device does not support the command
natively then LIO will perform the operations needed. It does this for
other commands like write same too. So, even though rbd does not
currently support these commands, you can use LIO and rbd and it will
still execute them although it would not be offloaded like how you would
want. For example, for write same LIO will just do multiple writes to
the rbd device.