[Lustre-discuss] programmatic access to parameters

[Andreas Dilger]

On 2010-03-24, at 18:39, burlen wrote:
> System limits are sometimes provided in a header, I wasn't sure if  
> Lustre adopted that approach.

Well, having static limits in a header doesn't help if the limits are  
dynamic.

> 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?

There isn't directly such a function, but indirectly this is possible  
to get from userspace without changing Lustre or the liblustreapi  
library:

/* Return the number of OSTs configured for the filesystem on which the
  * file descriptor "fd" is opened.
  * Return +ve number of OSTs on success, or -ve errno on failure. */
int get_ost_count(int fd)
{
         int ost_count = 0;
         int rc;

         rc = llapi_lov_get_uuids(fd, NULL, &ost_count);
         if (rc < 0)
                 return rc;

         return ost_count;
}


/* return the maximum possible number of OSTs any file can be striped  
over
  * for the filesystem on which the file descriptor "fd" is opened.
  * Return +ve max_stripe_count on success, or -ve errno on failure. */
int get_max_stripe_count(int fd)
{
         int max_stripe_count = 0;
         int rc;

         rc = llapi_lov_get_uuids(fd, NULL, &max_stripe_count);
         if (rc < 0)
                 return rc;

         if (max_stripe_count > LOV_MAX_STRIPE_COUNT)
                 max_stripe_count = LOV_MAX_STRIPE_COUNT;

         return max_stripe_count;
}

Note that there is a drawback from forcing a file to have N stripes,  
vs. letting Lustre make this decision.  If you request N stripes, but  
e.g. one OST is unavailable (full, offline, whatever) your file  
creation will fail.  If Lustre is making this decision it will use the  
currently available OSTs, regardless of how many there are configured.

> 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?

Sure, but not many applications run in this mode.  Either they have  
1:1 (file per process), N:M (shared single file, maximally-striped) or  
M:M (shared single file, maximally-striped, 1 process writing per  
stripe).

> 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.
>>
>


Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.