Tuning NFS client write pagecache

[Matthew Hodgson <matthew@mxtelecom.com>]

Hi all,

Is there any way to tune the linux NFSv3 client to prefer to write data 
straight to an async-mounted server, rather than having large writes to 
a file stack up in the local pagecache before being synced on close()?

I have an application which (stupidly) expects system calls to return 
fairly rapidly, otherwise an application-layer timeout occurs.  If I 
write (say) 100MB of data to an NFS share with the app, the write()s 
return almost immediately as the local pagecache is filled up - but then 
close() blocks for several minutes as the data is synced to the server 
over a slowish link.  Mounting the share as -o sync fixes this, as does 
opening the file O_SYNC or O_DIRECT - but ideally I want to generally 
encourage the client to flush a bit more aggressively to the server 
without the performance hit of making every write explicitly synchronous.

Is there a way to cap the size of pagecache that the NFS client uses?

This is currently on a 2.6.18 kernel (Centos 5.5), although I'm more 
than happy to use something less prehistoric if that's what it takes.

M.

-- 
Matthew Hodgson
Development Program Manager
OpenMarket | www.openmarket.com/europe
matthew.hodgson@openmarket.com