[Pvfs2-developers] Distribution by hostname

[Walter B. Ligon III]

I tend to agree with Pete and Sam ... Given the existing distro 
interface this is the right place to set a list of specific servers - 
though I tend to think it should be independent of the distro type (just 
as the number of servers is currently independent of which distro type 
you use).  The list of servers shouldn't be saved as part of the 
metadata - I think Sam's example of setting such a list on a directory 
is a distinct case.  In this case you set an extended attribute which is 
used as the default during file creation - but that list still isn't 
saved with the files (but might be saved as an extended attrib on a 
subdir ... but I digress).

On the other hand, if we had a good generic hint mechanism we COULD 
replace the entire distro interface and just use hints to do all of 
that.  I'm not sure this is the best approach because it is awkward. 
Especially if everything is specified using strings.

Maybe we should do it all with XML!   ;-)

Walt

Sam Lang wrote:
> 
> On Oct 7, 2006, at 2:09 PM, Rob Ross wrote:
> 
>> I agree completely with Pete. I think we might consider just  
>> adjusting the input parameters on the client side to allow for  
>> inclusion of the list of aliases. There is no reason for these to  be 
>> stored as part of the distribution information on the server, as  once 
>> the objects are created we already know where they are.
> 
> 
> I agree storing them in a file's distribution is redundant.  The  
> distribution stored on a directory in the extended attribute is the  
> only reason I can think of for storing the list in the distro.
> 
> In terms of interfaces, the distribution parameter we pass into  create 
> is really a sort of hint:  We don't require it (defaulting to  simple 
> stripe if its not specified), its opaque to the caller  (specified by a 
> key string: "varstrip" and arbitrary parameters), and  it functions as a 
> 'hint' in the way we seem to have a need for them,  changing the 
> distribution of that file.
> 
> It might be possible to design a hints interface that both allows us  to 
> express a distribution as one of possibly many generic hints being  
> passed into create, without losing the convenience of just passing in  a 
> distribution, or a list of servers.
> 
> -sam
> 
>>
>> We have what, 3 distributions that will need to be adjusted to this  
>> new scheme? That shouldn't be too bad, and they overall cover a  very 
>> wide range of options.
>>
>> Regards,
>>
>> Rob
>>
>> Pete Wyckoff wrote:
>>
>>> slang at mcs.anl.gov wrote on Fri, 06 Oct 2006 16:33 -0500:
>>>
>>>> On Oct 6, 2006, at 1:48 PM, Julian Martin Kunkel wrote:
>>>>
>>>>> Also it will not
>>>>> allow to set the servers for all distributions...
>>>>
>>>> Yeah I can't imagine wanting to ever do that.  It would mean  
>>>> passing  in a distribution different from the default simple- 
>>>> stripe, as well  as a hint saying you want a specific set of  
>>>> servers in the same  call.  Seems sort of yucky to me.  I'd  rather 
>>>> have all the  information about the distribution in the  
>>>> distribution.  You're even  able to use the distribution field in  
>>>> the directory hints structure  to specify per-directory IO server  
>>>> lists.  Not that you would ever  want to do that either...
>>>
>>> I agree with Sam that this is yucky.  I'm hijacking this thread.
>>> Let's forget about hints for a moment and decide how we want to
>>> extend the concept of distributions, as seen by users, in such a way
>>> that they can specify particular IO servers by name.  If this is an
>>> interface people want, we should design it properly, not just
>>> implement it with hints because we (might) have them.
>>> Some issues, please suggest approaches and other issues.  (I'm using
>>> "name" here to mean host alias.)
>>> 1.  What kind of control do users want?
>>>     - all data on one server by name?
>>>     - arbirtrary control of stripe sizes and host names?
>>> 2.  New distribution name, or extension to existing ones?
>>>     - dist-varstrip has a lot of flexibility, but no hostnames
>>>     - maybe a new "dist-single-host-by-name" is all that is desired
>>> 3.  Store hostnames in on-disk distribution?
>>>     - guessing no for the single-stripe distro, but perhaps somebody
>>>     can really think of a use case for this?
>>> 4.  User API
>>>     - through PVFS_dist_create
>>>     - (please not through both PVFS_dist_create + some hint)
>>>     - via environment variable too?
>>> If our design happens to end up as something that would be
>>> implemented well by hints, then we can think about using them.  For
>>> now, let's just get the design correct.
>>> We can come back and argue the merits of a generic hint interface in
>>> a different thread.
>>>         -- Pete
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>>
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-- 
Dr. Walter B. Ligon III
Associate Professor
ECE Department
Clemson University