[LU-12585] obdfilter read_bytes stat reports requested read bytes and not actual read bytes

• Status: Resolved
• Project: Lustre
• Affects Version/s: Lustre 2.10.7, Lustre 2.12.1
• Fix Version/s: Lustre 2.15.0
• Type: Bug
• Priority: Minor
• Reporter: Aurelien Degremont (Inactive)
• Assignee: Patrick Farrell
• Resolution: Fixed
• Severity: 3

Description

obdfilter.*.stats has a read_bytes metric. This metrics reports the number of requested bytes, and not what I was expecting, what was actually read bytes from the OST.

By example, if you have a 1MB file, and you try to read 16MB from it, the{{ read_bytes}} will likely reports 16MB and not 1MB.

To reproduce:

# llmount.sh

# dd if=/dev/zero of=/mnt/lustre/dump bs=1M count=1
# lctl set_param -n ldlm.namespaces.*.lru_size=clear

# dd if=/mnt/lustre/dump of=/dev/null bs=16M count=1
# lctl get_param obdfilter.*.stats | grep read_bytes
read_bytes 4 samples [bytes] 4194304 4194304 16777216
 

I think this comes from the fact that the stats is updated at I/O preparation, and not I/O completion.

Is this intended? I think we should either:

• Fix the stat to store the actual read bytes
• Or, add a second stat to store the actual read bytes if we don't want to change this one

Issue Links

• is related to: LU-15642 restore server read/write latency mea...

Comments

[Andreas Dilger]

in newer Lustre releases, there is:

osd-ldiskfs.myth-OST0000.stats=
snapshot_time             1614211618.617166153 secs.nsecs
get_page                  92759 samples [usec] 0 103 426203 4154059
cache_access              20606 samples [pages] 1 1024 16346835
cache_hit                 4869 samples [pages] 1 1024 342396
cache_miss                20005 samples [pages] 1 1024 16004439

which shows whether pages are coming from cache or from disk.

That said, with patch https://review.whamcloud.com/38816 "LU-13597 ofd: add more information to job_stats" the read stats are now stored after all of the pages have been read, so it would be trivial to replace "tot_bytes" with an actual counter of the bytes read.

[Gerrit Updater]

"Patrick Farrell <[e-mail]>" uploaded a new patch: https://review.whamcloud.com/46075
Subject: LU-12585 obdfilter: Use actual I/O bytes in stats
Project: fs/lustre-release
Branch: master
Current Patch Set: 1
Commit: 34cbc31a9e73498d7f9324e942c85fbc44791c83

[Gerrit Updater]

"Patrick Farrell <[e-mail]>" uploaded a new patch: https://review.whamcloud.com/46229
Subject: LU-12585 mdt: Add read/write latency to MDT stats
Project: fs/lustre-release
Branch: master
Current Patch Set: 1
Commit: 6af42177e90e323d8d53878db3e2f56dd096b0ab

[Gerrit Updater]

"Oleg Drokin <[e-mail]>" merged in patch https://review.whamcloud.com/46075/
Subject: LU-12585 obdfilter: Use actual I/O bytes in stats
Project: fs/lustre-release
Branch: master
Current Patch Set:
Commit: 1c98e950fa7c9fac3d7494278abebee7c64c5397

[Gerrit Updater]

"Oleg Drokin <[e-mail]>" merged in patch https://review.whamcloud.com/46229/
Subject: LU-12585 mdt: Add read/write latency to MDT stats
Project: fs/lustre-release
Branch: master
Current Patch Set:
Commit: a942fc916871eebe6615fe8e29471e2386d46f1d

[Peter Jones]

Landed for 2.15