[Lustre-devel] read ahead

[Peter Braam <Peter.Braam@Sun.COM>]

Tom.Wang wrote:
> Hi, Peter
>
> I just talked with Matt about his pCIFS test. In his test, there are 2 
> clients, 2 OST, (stripe_count =2, stripe_size=1M).
> Each client run 1 thread to read a shared file. Each thread(client) 
> will only read data of the shared file from 1 OST, so
> read request is discontiguous (1M step stride I/O) for each of thread, 
> which will not trigger read-ahead,  because
> read-ahead window will be reset once it met discontinuous  read  
> request.  So  kept  read-ahead in the same step
> of stride step ((stripe_count -1) * stripe_size) instead of 
> contiguous  could just fix this problem, and it is what we are
> working now.
>
Finally!  This makes a lot of sense.

> Btw:  we also need this in lustre collective read  ADIO driver,  
> since  we  will also  need  reorganize the  data  as  1client-> 1OST 
>          model  like pCIFS does.

I now see why this stripe behavior with read-ahead is so very important 
for Lustre in some cases.

It would also be important to know that in basic cases we really get as 
good read performance as write.

- Peter -

>          Another alternative way maybe move read-ahead mechanism to 
> osc layer, but maybe a bad idea.
> Thanks
> WangDi
>