Lustre discuss - Jun 2006 - aio asynchronous I/O

On Jun 06, 2006  21:56 -0400, Hank Jakiela wrote:
> Does Lustre support asynchronous I/O calls such as aio_read()?
Not currently.  While Lustre does perform async reads in order to do
readahead, this functionality isn''t exported via the AIO interface yet.
> I have an application that does small random reads from a very large 
> file.  Each read incurs a disk access latency.  I''d like to hide
the
> latency as much as possible by queuing up a number of reads so that the 
> seeks can be overlapped.  The file system has 12 OSTs and over 100 disks, 
> so there is opportunity for parallelism.
> 
> When I run my test program against an NFS mount (without Lustre) there is 
> an obvious increase in I/O rate as the number of outstanding reads 
> increases, but with Lustre I don''t see any difference. 
It''s as if each
> async read is actually being handled synchronously.  Am I missing
something?
No.  The plumbing to do async reads exists in the 2.6 kernel and in Lustre,
it just hasn''t been connected yet.  We''d welcome a patch to do
so.

Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.

Does Lustre support asynchronous I/O calls such as aio_read()?

I have an application that does small random reads from a very large 
file.  Each read incurs a disk access latency.  I''d like to hide the 
latency as much as possible by queuing up a number of reads so that the 
seeks can be overlapped.  The file system has 12 OSTs and over 100 disks, 
so there is opportunity for parallelism.

When I run my test program against an NFS mount (without Lustre) there is 
an obvious increase in I/O rate as the number of outstanding reads 
increases, but with Lustre I don''t see any difference.  It''s
as if each
async read is actually being handled synchronously.  Am I missing something?

Thanks
Hank


_______________________________________________________________
Hank Jakiela                                hank.jakiela@hp.com
Hewlett-Packard Company     High Performance Computing Division
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