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

Related — Sat, 12 Mar 2022 05:11:25 +0000

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

Whamcloud Community JIRA

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