[Pvfs2-developers] [PATCH] modify twod-stripe's default params and add some documentation

[Sam Lang]

On Jul 7, 2008, at 10:57 AM, Phil Carns wrote:

> Sam Lang wrote:
>> On Jul 7, 2008, at 10:24 AM, Phil Carns wrote:
>>> There are a couple of options for dealing with the case where  
>>> num_groups > num_dfiles if we want to keep the current defaults:
>>>
>>> 1) Transparently reduce the num_groups within the distribution  
>>> calculation.  From the user's perspective the params wouldn't  
>>> change; the distribution would just automatically fall back to  
>>> acting like simple stripe.  We would just need to add this to the  
>>> top of a few of the dist functions:
>>>
>>> if(num_groups > server_ct)
>>> {
>>>  num_groups = server_ct;
>>> }
>>>
>>> 2) Return an explicit error at create time.  The varstrip  
>>> distribution has this behavior now; it actually prevents you from  
>>> creating a file if the dfile count isn't plausible for the  
>>> distribution parameters.  The way to do this is to implement the  
>>> get_num_dfiles() function in twod_stripe so that it can perform a  
>>> safety check and return an error at create time if the user tries  
>>> to use too high of a num_groups setting. On the command line you  
>>> would get a generic errno (EINVAL or whatever) but we could put a  
>>> descriptive message in the logs.
>>>
>>> Option 1) seems reasonable to me; it is a small code change, we  
>>> get to keep the existing desirable default parameters, and the  
>>> single server case just transparently falls back to simple stripe.
>> There's a third option, and what I've been proposing -- instead of  
>> changing the actual parameter, adjust the value during the  
>> logical_to_physical calculation.
>> -sam
>
> No, that's actually the same thing I was proposing, I just didn't  
> realize that you meant the same thing until just now :)  The code  
> snippet above would not change the parameter in the directory hints.  
> "num_groups" is a variable within the logical_to_physical_offset()  
> function.

Lets go with that then.
-sam

>
>
> -Phil