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

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

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 measurements