[Ocfs2-users] Diagnosing poor write performance

[Eric Ren]

Hi,

On 03/31/2016 01:52 PM, Graeme Donaldson wrote:
> On 31 March 2016 at 04:17, Eric Ren <zren at suse.com> wrote:
>
>> 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.
>
>
> Here is all the output requested: http://pastebin.com/raw/BnJAQv9T

Paste directly in email is fine, that way all info would be archived;-)

Hah, your disk is too small for ocfs2. What's your using scenario? it 
does really really matters. I guess files on ocfs2 volume are usually 
small ones? If so, 4k block size and 4k cluster size is little big? For
more info to get optimal format parameter, please see mkfs.ocfs2 
manuall, user maillist or google for the topic about ocfs2 formatting. 
I'm not good at it.

>
> It's interesting to me that you guessed the usage is over 60%. It is indeed
> sitting at 65%. Is the solution as simple as ensuring that an OCFS2
> filesystem doesn't go over the 60% usage mark? Or am I getting ahead of
> myself a little?

Yes, so use ocfs2 on top cLVM is good idea if you want it to get 
resilience. I'm not sure if tune.ocfs2 can change block size suchlike 
offline. FWIW, fragmentation is always evil;-)

Eric

>
> Thanks for your effort so far!
>
> Graeme.
>