On 5/29/24 12:48 AM, Damien Le Moal wrote:
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.
QUEUE_FLAG_NOMERGES can also be set through sysfs (see also
queue_nomerges_store()). This is one of the reasons why using the merge
infrastructure for combining REQ_OP_COPY_DST and REQ_OP_COPY_SRC is
unacceptable.
Thanks,
Bart.
