----- Original Message ---- > From: Trond Myklebust <trond.myklebust@xxxxxxxxxx> > To: Martin Knoblauch <spamtrap@xxxxxxxxxxxx> > Cc: linux-nfs list <linux-nfs@xxxxxxxxxxxxxxx> > Sent: Tuesday, April 15, 2008 6:12:32 PM > Subject: Re: RFC/Patch: Make NFS Readahead tunable > > > On Tue, 2008-04-15 at 06:39 -0700, Martin Knoblauch wrote: > > Hi, > > > > while tracking down a very interesting interaction between Sun/SAM-FS and > Linux NFS clients, we found out that the value of NFS_MAX_READ_AHEAD is to > agressive/big for the specific use-case. > > > > For testing, instead of always recompiling the kernel with different values, > I came up with the following patch. It introduces a tunable > "/proc/sys/fs/nfs/nfs_ra_factor" with possible values between 0-15. > > > > Not sure whether it is actually a good thing to have. Better would be to set > the read-ahead factor per filesystem via a mount option. > > > > The patch is against 2.6.24. It applies with offsets against 2.6.25-rc9. In > case my mail client messes up the whitespace, the patch is also attached. > > NFS_MAX_READ_AHEAD just sets an upper limit on the standard vfs/mm-level > page readahead algorithm. If you need finer control over that readahead > algorithm, then the kernel already has full support for the > posix_fadvise() system call. > But to use posix_fadvise I would need control over the application. Correct? This is not given in the case I am actually dealing with. The NFS-server is Solaris-10/Sparc/SAM-FS. It turns out that *any* access to an "offline" file from a Linux-NFS client messes (and I mean messes) up the performance of the stager process if the read-ahead window is larger that 2 or three time rsize. Now - I have a pretty clear opinion of "who is at fault", but doing a band-aid on the Linux side may just be the easier thing, at least short/mid-term. Not sure how important the end-customer really is to the makers of SAM-FS. In general, of course controlling the readahead behaviour from the application is the better solution. anyway, the patch is simple enough to carry around. Martin > Cheers > Trond > > > -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
