On 27/11/24 03:14PM, Martin K. Petersen wrote:
Bart,
Submitting a copy operation as two bios or two requests means that
there is a risk that one of the two operations never reaches the block
driver at the bottom of the storage stack and hence that a deadlock
occurs. I prefer not to introduce any mechanisms that can cause a
deadlock.
How do you copy a block range without offload? You perform a READ to
read the data into memory. And once the READ completes, you do a WRITE
of the data to the new location.
Token-based copy offload works exactly the same way. You do a POPULATE
TOKEN which is identical to a READ except you get a cookie instead of
the actual data. And then once you have the cookie, you perform a WRITE
USING TOKEN to perform the write operation. Semantically, it's exactly
the same as a normal copy except for the lack of data movement. That's
the whole point!
Martin
This approach looks simpler to me as well.
But where do we store the read sector info before sending write.
I see 2 approaches here,
1. Should it be part of a payload along with write ?
We did something similar in previous series which was not liked
by Christoph and Bart.
2. Or driver should store it as part of an internal list inside
namespace/ctrl data structure ?
As Bart pointed out, here we might need to send one more fail
request later if copy_write fails to land in same driver.
Thanks,
Nitesh Shetty
