[LU-15642] restore server read/write latency measurements

• Type: Bug
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.16.0
• Affects Version/s: Lustre 2.15.0
• Severity: 3
• Assignee: Andreas Dilger
• Reporter: Andreas Dilger

Description

The patch https://review.whamcloud.com/46075 "LU-12585 obdfilter: Use actual I/O bytes in stats" changed the measurement of the read/write latency stats, so that they now contain the network round-trip time, while they previously only contained the local filesystem IO time. This caused the reported latency with 46076 applied to be much higher (tens or hundreds of milliseconds) than the pre-patch latency (hundreds of microseconds for flash).

While it may be necessary to account for the actual read/write bytes after the RPC transfer is complete, the code should account for the IO latency after the IO is complete, as it did before, rather than after the RPC is complete. The RPC stats at the OST level and on the client will include the full RPC latency, and the ofd stats should only account for the storage latency.

Issue Links

• is related to: LU-16110 Make output of jobs_stats and rename_stats valid YAML
• is related to: LU-16231 Lustre stats header incorrectly using boot time
• is related to: LU-12585 obdfilter read_bytes stat reports requested read bytes and not actual read bytes