[Lustre-discuss] programmatic access to parameters

burlen burlen.loring at gmail.com
Wed Mar 24 17:39:00 PDT 2010


System limits are sometimes provided in a header, I wasn't sure if 
Lustre adopted that approach. The llapi_* functions are great, I see how 
to set the stripe count and size. I wasn't sure if there was also a 
function to query about the configuration, eg number of OST's deployed?

This would be for use in a global hybrid megnetospheric simulation that 
runs on a large scale (1E4-1E5 cores). The good striping parameters 
depend on the run, and could be calculated at run time. It can make a 
significant difference in our run times to have these set correctly. I 
am not sure if we always want a stripe count of the maximum. I think 
this depends on how many files we are synchronously writing, and the 
number of available OST's total. Eg if there are 256 OST's on some 
system and we have 2 files to write would it not make sense to set the 
stripe count to 128?

We can't rely on our user to set the Lustre parameter correctly. We 
can't rely on the system defaults either, they typically aren't set 
optimally for our use case. MPI hints look promising but the ADIO Lustre 
optimization are fairly new,  as far as I understand not publically 
available in MPICH until next release (maybe in may?). We run on a 
variety of systems some with variety of MPI implementation (eg Cray, 
SGI). The MPI hints will only be useful on implementation that support 
the particular hint. From a consistency point of view we need to both 
make use of MPI hints and direct access via the llapi so that we run 
well on all those systems, regardless of which MPI implementation is 
deployed.  

Thanks
Burlen


Andreas Dilger wrote:
> On 2010-03-23, at 14:25, burlen wrote:
>> How can one programmatically probe the lustre system an application is
>> running on?
>
> Lustre-specific interfaces are generally "llapi_*" functions, from 
> liblustreapi.
>
>> At compile time I'd like access to the various lustre system limits ,
>> for example those listed in ch.32 of operations manual.
>
> There are no llapi_* functions for this today.  Can you explain a bit 
> better what you are trying to use this for?
>
> statfs(2) will tell you a number of limits, as will pathconf(3), and 
> those are standard POSIX APIs.
>
>> Incidentally one I didn't see listed in that chapter is the maximum 
>> number of OST's a single file can be striped across.
>
> That is the first thing listed:
>
>>> 32.1    Maximum Stripe Count
>>> The maximum number of stripe count is 160. This limit is hard-coded, 
>>> but is near the upper limit imposed by the underlying ext3 file 
>>> system. It may be increased in future releases. Under normal 
>>> circumstances, the stripe count is not affected by ACLs.
>>>
>> At run time I'd like to be able to probe the size (number of OSS, OST
>> etc...) of the system the application is running on.
>
>
> One shortcut is to specify "-1" for the stripe count will stripe a 
> file across all available OSTs, which is what most applications want, 
> if they are not being striped over only 1 or 2 OSTs.
>
> If you are using MPIIO, the Lustre ADIO layer can optimize these 
> things for you, based on application hints.
>
> If you could elaborate on your needs, there may not be any need to 
> make your application more Lustre-aware.
>
> Cheers, Andreas
> -- 
> Andreas Dilger
> Sr. Staff Engineer, Lustre Group
> Sun Microsystems of Canada, Inc.
>




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