[Lustre-discuss] brief 'hangs' on file operations

[Andreas Dilger]

It could be both. We had good success with performance improvements up to 1GB journals, and AFAIK Some customers had reduced their journal size to 128 MB without significant performance impact to reduce RAM consumption on their OSS. We haven't really made any testing with reduced journal size to reduce latency, so I don't know what performance you will see. The minimum possible size is about 6MB, which would unfortunately single-thread all of your IO and probably give lousy performance unless the journal was on an SSD. 

Note that the amount of RAM used by the kernel journalling code can equal the size of the journal, so don't make it too large for your system. 

That said, my speculation that this stall is caused by the journal is still only speculation. You should check the /proc/fs/nbd/{dev}/history file to see the commit timings of recent transactions.  

Cheers, Andreas

On 2010-09-03, at 4:04, Tina Friedrich <Tina.Friedrich at diamond.ac.uk> wrote:

> And another quick question - would this be more likely to be the journal on the MDS, or the OSS servers?
> 
> On 02/09/10 17:38, Tina Friedrich wrote:
>> Hello,
>> 
>> On 02/09/10 17:28, Tina Friedrich wrote:
>>> Hi Andreas,
>>> 
>>> thanks for your answer.
>>> 
>>>>> Causing most grieve at the moment is that we sometimes see delays
>>>>> writing files. From the writing clients end, it simply looks as if I/O
>>>>> stops for a while (we've seen 'pauses' of anything up to 10 seconds).
>>>>> This appears to be independent of what client does the writing, and
>>>>> software doing the writing. We investigated this a bit using strace and
>>>>> dd; the 'slow' calls appear to always be either open, write, or close
>>>>> calls. Usually, these take well below 0.001s; in around 0.5% or 1% of
>>>>> cases, they take up to multiple seconds. It does not seem to be
>>>>> associated with any specific OST, OSS, client or anything; there is
>>>>> nothing in any log files or any exceptional load on MDS or OSS or
>>>>> any of
>>>>> the clients.
>>>> 
>>>> This is most likely associated with delays in committing the journal
>>>> on the MDT or OST, which can happen if the journal fills completely.
>>>> Having larger journals can help, if you have enough RAM to keep them
>>>> all in memory and not overflow. Alternately, if you make the journals
>>>> small it will limit the latency, at the cost of reducing overall
>>>> performance. A third alternative might be to use SSDs for the journal
>>>> devices.
>>> 
>>> Just to double check - that would be the file system journal, I assume?
>>> 
>>> That makes a lot of sense; is there a way to verify that this is the
>>> issue we're having?
>>> 
>>> Journal size appears to be 400M - if we were to try increasing it, how
>>> would be determine what to best set it to?
>> 
>> That was meant to be 'if we were to try increasing or decreasing it' -
>> sounds to us as if decreasing might be the better option (as in, if this
>> is the journal flushing, having less journal to flush would probably be
>> better - or is that the wrong idea?)
>> 
>> 
>>>>> The other issue is that we frequently see delays when trying to read a
>>>>> file. I sometimes takes more than 60s for a file to be visible on a
>>>>> machine after the initial write on a different machine has completed
>>>>> (both machines being Lustre clients). Again, there is nothing in the
>>>>> logs, nor exceptional load on any of the machines.
>>>> 
>>>> This is probably just a manifestation of the first problem. The issue
>>>> likely isn't in the read, but a delay in flushing the data from the
>>>> cache of the writing client. There were fixes made in 1.8 to increase
>>>> the IO priority for clients writing data under a lock that other
>>>> clients are waiting on.
>>> 
>>> We kind of suspected them to be related, yes.
>>> 
>>> Tina
>>> 
>> 
>> 
> 
> 
> -- 
> Tina Friedrich, Computer Systems Administrator, Diamond Light Source Ltd
> Diamond House, Harwell Science and Innovation Campus - 01235 77 8442