On Wed, Sep 17, 2008 at 11:23 AM, Martin Knoblauch <knobi@xxxxxxxxxxxx> wrote: > ----- Original Message ---- > >> From: Chuck Lever <chucklever@xxxxxxxxx> >> To: Peter Staubach <staubach@xxxxxxxxxx> >> Cc: Martin Knoblauch <knobi@xxxxxxxxxxxx>; linux-nfs list <linux-nfs@xxxxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx >> Sent: Wednesday, September 17, 2008 5:41:15 PM >> Subject: Re: [RFC][Resend] Make NFS-Client readahead tunable >> >> On Wed, Sep 17, 2008 at 9:06 AM, Peter Staubach wrote: >> > Martin Knoblauch wrote: >> >> >> >> Hi, >> >> >> >> the following/attached patch works around a [obscure] problem when an 2.6 >> >> (not sure/caring about 2.4) NFS client accesses an "offline" file on a >> >> Sun/Solaris-10 NFS server when the underlying filesystem is of type SAM-FS. >> >> Happens with RHEL4/5 and mainline kernels. Frankly, it is not a Linux >> >> problem, but the chance for a short-/mid-term solution from Sun are very >> >> slim. So, being lazy, I would love to get this patch into Linux. If not, I >> >> just will have to maintain it for eternity out of tree. >> >> >> >> The problem: SAM-FS is Suns proprietary HSM filesystem. It stores >> >> meta-data and a relatively small amount of data "online" on disk and pushes >> >> old or infrequently used data to "offline" media like e.g. tape. This is >> >> completely transparent to the users. If the date for an "offline" file is >> >> needed, the so called "stager daemon" copies it back from the offline >> >> medium. All of this works great most of the time. Now, if an Linux NFS >> >> client tries to read such an offline file, performance drops to "extremely >> >> slow". After lengthly investigation of tcp-dumps, mount options and >> >> procedures involving black cats at midnight, we found out that the readahead >> >> behaviour of the Linux NFS client causes the problem. Basically it seems to >> >> issue read requests up to 15*rsize to the server. In the case of the >> >> "offline" files, this behaviour causes heavy competition for the inode lock >> >> between the NFSD process and the stager daemon on the Solaris server. >> >> >> >> - The real solution: fixing SAM-FS/NFSD interaction. Sun engineering acks >> >> the problem, but a solution will need time. Lots of it. >> >> - The working solution: disable the client side readahead, or make it >> >> tunable. The patch does that by introducing a NFS module parameter >> >> "ra_factor" which can take values between 1 and 15 (default 15) and a >> >> tunable "/proc/sys/fs/nfs/nfs_ra_factor" with the same range and default. >> > >> > Hi. >> > >> > I was curious if a design to limit or eliminate read-ahead >> > activity when the server returns EJUKEBOX was considered? >> > Unless one can know that the server and client can get into >> > this situation ahead of time, how would the tunable be used? >> >> I tend to agree. A tunable is probably not a good solution in this case. >> >> I would bet that this lock contention issue is a problem in other more >> common cases, and would merit some careful analysis. >> > > Are you talking wrt. a Solaris NFS-Server with SAM-FS/QFS as backend filesystem? I misread your mail, and thought the inode lock contention issue was on the client. -- Chuck Lever -- 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
