[Pvfs2-developers] null-aio

[Sam Lang]

Hi Phil,

Its good to get this functionality into the code base -- we've had a  
number of attempts at this sort of thing, but none of them got  
committed to HEAD, and having it in there in whatever state is better  
than not.  I have a concern (and overall design gripe) with the use of  
AIO interfaces for this sort of thing, when we already have callback  
structures in dbpf.

We now have two levels of indirection, with threads being created and  
managed in both.  Obviously, that's more code to manage in different  
locations that do more or less the same thing, making it harder for  
others to understand and augment.

In general I don't think the aio callback structures are needed at  
all, but its admittedly much easier to implement to those functions  
than the dbpf ones, if only because of the disorderly op mgmt code in  
dbpf bstream.

I don't know what our long term plans will be for the trove code, but  
I would vote for trying to move towards a simpler centralized location  
for management of the IO threads and queues, and different callbacks  
for IO impls.  I've done a prototype of this for queue/thread  
management and O_DIRECT, and I think it would clean things up quite a  
bit to go that route.

-sam

On Apr 17, 2008, at 4:10 PM, Phil Carns wrote:

> There is a new trove method available in trunk now called "null- 
> aio". It can be selected by putting "TroveMethod null-aio" in the  
> <StorageHints> section of the file system configuration file.
>
> This is only useful for debugging purposes, because it deliberately  
> skips doing any file I/O on the server side.  Please use with  
> caution! It does all metadata operations the same as any other  
> method, but file reads will return garbage and file writes are  
> thrown away.  Writing beyond eof triggers a truncate to mimic the  
> appropriate resulting bstream size.
>
> This might be useful once in a while for narrowing down performance  
> problems between network and storage.  It takes the storage out of  
> the loop and shows approximately what the network is capable of by  
> itself. Of course it will only work for benchmarks that don't verify  
> data correctness (or otherwise rely on data read off of PVFS).
>
> We used to have a compile time option (--disable-disk-io) for this  
> same purpose, but that actually hasn't worked in a while.  Nowadays  
> its easier to just do this as a trove method that can be selected at  
> runtime without recompiling.
>
> -Phil
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