[dm-devel] [PATCH 3/7] block: copy offload support infrastructure

[Douglas Gilbert]

On 2021-08-17 4:41 p.m., Mikulas Patocka wrote:
> 
> 
> On Tue, 17 Aug 2021, Bart Van Assche wrote:
> 
>> On 8/17/21 3:14 AM, SelvaKumar S wrote:
>>> Introduce REQ_OP_COPY, a no-merge copy offload operation. Create
>>> bio with control information as payload and submit to the device.
>>> Larger copy operation may be divided if necessary by looking at device
>>> limits. REQ_OP_COPY(19) is a write op and takes zone_write_lock when
>>> submitted to zoned device.
>>> Native copy offload is not supported for stacked devices.
>>
>> Using a single operation for copy-offloading instead of separate operations
>> for reading and writing is fundamentally incompatible with the device mapper.
>> I think we need a copy-offloading implementation that is compatible with the
>> device mapper.
> 
> I once wrote a copy offload implementation that is compatible with device
> mapper. The copy operation creates two bios (one for reading and one for
> writing), passes them independently through device mapper and pairs them
> at the physical device driver.
> 
> It's here: http://people.redhat.com/~mpatocka/patches/kernel/xcopy/current

In my copy solution the read-side and write-side bio pairs share the same 
storage (i.e. ram) This gets around the need to copy data between the bio_s.
See:
    https://sg.danny.cz/sg/sg_v40.html
in Section 8 on Request sharing. This technique can be efficiently extend to
source --> destination1,destination2,...      copies.

Doug Gilbert

> I verified that it works with iSCSI. Would you be interested in continuing
> this work?
> 
> Mikulas
> 
>> Storing the parameters of the copy operation in the bio payload is
>> incompatible with the current implementation of bio_split().
>>
>> In other words, I think there are fundamental problems with this patch series.
>>
>> Bart.
>>
>