Re: [PATCH v20 02/12] Add infrastructure for copy offload in block and request layer.

[Damien Le Moal <dlemoal@kernel.org>]

On 5/29/24 15:17, Nitesh Shetty wrote:
> On 24/05/24 01:33PM, Bart Van Assche wrote:
>> On 5/20/24 03:20, Nitesh Shetty wrote:
>>> We add two new opcode REQ_OP_COPY_DST, REQ_OP_COPY_SRC.
>>> Since copy is a composite operation involving src and dst sectors/lba,
>>> each needs to be represented by a separate bio to make it compatible
>>> with device mapper.
>>> We expect caller to take a plug and send bio with destination information,
>>> followed by bio with source information.
>>> Once the dst bio arrives we form a request and wait for source
>>> bio. Upon arrival of source bio we merge these two bio's and send
>>> corresponding request down to device driver.
>>> Merging non copy offload bio is avoided by checking for copy specific
>>> opcodes in merge function.
>>
>> In this patch I don't see any changes for blk_attempt_bio_merge(). Does
>> this mean that combining REQ_OP_COPY_DST and REQ_OP_COPY_SRC will never
>> happen if the QUEUE_FLAG_NOMERGES request queue flag has been set?
>>
> Yes, in this case copy won't work, as both src and dst bio reach driver
> as part of separate requests.
> We will add this as part of documentation.

So that means that 2 major SAS HBAs which set this flag (megaraid and mpt3sas)
will not get support for copy offload ? Not ideal, by far.

-- 
Damien Le Moal
Western Digital Research