[Ocfs2-users] Diagnosing poor write performance

[Eric Ren]

Hi,

>>
>> How did you perform the testing? It really matters. If you write a file on
>> shared disk from one node, and read this file from another node, without,
>> or with very little interval, the writing IO speed could decrease by ~20
>> times according my previous testing(just as a reference). It's a extremely
>> bad situation for 2 nodes cluster, isn't?
>>
>> But it's incredible that in your case writing speed drop by >3000 times!
>
>
> I simply used 'dd' to create a file with /dev/zero as a source. If there is
> a better way to do this I am all ears.

Alright, you just did a local IO on ocfs2, then the performance 
shouldn't be that bad. I guess the ocfs2 volume has been used over 60%? 
or seriously fragmented?
Please give info with `df -h`, and super block with debugfs.ocfs2, and 
also the exact `dd` command you performed. Additionally, perform `dd` on 
each node.

You know, ocfs2 is a shared disk fs. So 3 basic testing cases I can 
think of are:
1. only one node of cluster do IO;
2. more than one nodes of cluster perform IO, but each nodes just 
read/write its own file on shared disk;
3. like 2), but some nodes read and some write a same file on shared disk.

The above model is much theoretically simplified though. The practical 
scenarios could be much more complicated, like fragmentation issue that 
your case much likely is.


>> Could you firstly do test on LVM, then DRBD, and then OCFS2? Let's blame
>> on them more fairly.

>>
> If I do a similar write of a file to a directory that exists on a LVM LV I
> get roughly 100 megabytes/sec.
>
> I can't write straight to the DRBD device, as that would entail wiping the
> customer's OCFS2 filesystem, which I cannot do.

OK, it's product environment. I can understand.

Eric