ublk: Adding Latency Measurement Support to UBLK

ublk: Adding Latency Measurement Support to UBLK

[Yoav Cohen]

Dear Ming,

We would like to propose adding support to the ublk server for measuring latency at each stage of the I/O path. Currently, we are missing key information from the ublk driver—for example, the exact time a completion queue entry (CQE) was issued for an I/O operation, or when an I/O reached the ublk driver.

After reviewing the code, it seems this information is not currently accessible. One possible approach could be to extend the ublksrv_io_desc structure when a specific flag is passed during the creation of the ublk device (or by some other mechanism).

We are very interested in contributing this feature but would appreciate your thoughts and feedback before proceeding.

Thank you,
Yoav

Re: ublk: Adding Latency Measurement Support to UBLK

[Ming Lei]

On Thu, Jun 05, 2025 at 12:52:49PM +0000, Yoav Cohen wrote:
> Dear Ming,
> 
> We would like to propose adding support to the ublk server for measuring latency at each stage of the I/O path. Currently, we are missing key information from the ublk driver—for example, the exact time a completion queue entry (CQE) was issued for an I/O operation, or when an I/O reached the ublk driver.
> 
> After reviewing the code, it seems this information is not currently accessible. One possible approach could be to extend the ublksrv_io_desc structure when a specific flag is passed during the creation of the ublk device (or by some other mechanism).
> 
> We are very interested in contributing this feature but would appreciate your thoughts and feedback before proceeding.

One idea is to add some trace points which can be attached reliably by
ebpf program, then you can run any analysis on the collected data from
ebpf program.

Please see the block layer's example:

trace_block_rq_issue()
trace_block_rq_complete()
...


Thanks,
Ming