[OpenAFS] AFS namei file servers, SAN, any issues elsewhere? We've had some. Can AFS _cause_ SAN issues?

Robert Banz rob@nofocus.org
Thu, 13 Mar 2008 13:32:52 -0700

AFS can't really cause "san issues" in that it's just another  
application using your filesystem.  In some cases, it can be quite a  
heavy user of such, but since its only interacting through the fs, its  
not going to know anything about your underlying storage fabric, or  
have any way of targeting it for any more badness than any other  
filesystem user.

One of the big differences that would effect the filesystem IO load  
that occurred between 1.4.1 & 1.4.6 was the removal functions that  
made copious fsync operations.  These operations were called in  
fileserver/volserver functions that modified various in-volume  
structures, specifically file creations and deletions, and would lead  
to rather underwhelming performance when doing vos restores, deleting,  
or copying large file trees.  In many configurations, this causes the  
OS to pass on a call to the underlying storage to verify that all  
changes written have been written to *disk*, causing the storage  
controller to flush its write cache.  Since this defeats many of the  
benefits (wrt I/O scheduling) on your storage hardware of having a  
cache, this could lead to overloaded storage.

Some storage devices have the option to ignore these calls from  
devices, assuming your write cache is reliable.

Under UFS, I would suggest that you'd be running in 'logging' mode  
when using the namei fileserver on Solaris, as yes, fsck is rather  
horrible to run.  Performance on reasonably recent versions of ZFS  
were quite acceptable as well.

Anyhow, hope this is of some help.