[LU-333] Lustre client procfs stats: read_bytes does not record the number of bytes transfered from the fs.

Fri, 12 Aug 2011 15:19:00 +0000

Read bytes
----------

A client of a Lustre filesystem will be interested in the total bytes
transfered over the fabric. The stats file appears to provide a
valuable snapshot of high-level data transfer metrics. However, after
investigation the values recorded are of limited value. read_bytes
returns the number of bytes that have been requested. This is not the
same as the number of bytes that have been read. The example below
illustrates this confusion:

[root@rhel6_21 ~]# echo "hello lustre" > /mnt/lustre/test.txt
[root@rhel6_21 ~]# cat /mnt/lustre/test.txt > /dev/null
[root@rhel6_21 ~]# cat /proc/fs/lustre/llite/lustre-ffff88001aa95c00/stats
...
read_bytes                1 samples [bytes] 2097152 2097152 2097152
write_bytes               1 samples [bytes] 13 13 13
...

In this example, the read on the file was performed by cat. This
requests the number of bytes it needs to fill it's internal buffer. It
continues to do this until the read returns zero. So, in our example,
the internal buffer size of cat is 1KB and it performs two reads. As
it stands, this metric may be misleading to the uninformed.

Whamcloud Community JIRA

[LU-333] Lustre client procfs stats: read_bytes does not record the number of bytes transfered from the fs.