Trond Myklebust wrote:
On Thu, 2008-08-21 at 12:05 +1000, Shehjar Tikoo wrote:
By changing pdflush settings on the client, I'd be changing the
settings for the whole system. Is there a proc FS entry or any
other config param that lets me lower the number of write
requests buffered at client before the commit request is sent?
Write buffering is not something which is under the control of the
NFS filesystem: it is entirely managed by the VM. The filesystem
only enforces the close-to-open cache consistency requirements,
which of course are specific to NFS.
True, but I see that there are atleast two cases where NFS client will
send out the outstanding requests:
1. In nfs_flush_incompatible and through the balanced_rate_limited
code, which is essentially an interface into the VM.
2. In a few other places in VFS, where it sends the requests after
checking whether the file is opened O_SYNC or if the inode is IS_SYNC.
WRT point 2, if its possible to send the requests out explicitly for
these two flags, i.e. O_SYNC or S_SYNC, it might be possible to
optimize the transmit behaviour for large write() record sizes, which
are left uncommitted either:
1. till the full record has been transmitted as UNSTABLE.
2. or till the file is closed.
...resulting in a big flush at the server when the commit does come
in.
See the attached file for the plot of what I am observing. The plot is
based on data for 2.6.26 but I see the same behaviour on 2.6.27-rc3.
Thanks
Shehjar
