Re:You've bought the hype (Score 1)

Hi, Peter is right about SANs being overhyped in the past. However, the hacks for shared disk access he describes are now being replaced by real shared disk file systems like GFS (Linux -- http://www.globalfilesystem.org ), CXFS (SGI), and CFS (Compaq's Cluster File System for Digital UNIX). These file systems allow a SAN network with a homogeneous OS to share disks as if they were local. The other recent development with SANs is that they are starting to get cheaper (FC adapter cards less than $300, FC hubs less than $100 per port) and the drivers have become reliable and reasonably robust. SANs can be constructed from RAID disks, of course, and as your disk access or capacity requirements increase the idea is you can just keep adding new disks and/or machines to your network. My group at the University of Minnesota has written GFS with the help of several others in the Linux community, and with alot of feedback from Linux kernel hacking luminaries like Stephen Tweedie. Also GFS is currently the only GPL'ed shared disk file system availble for Linux. Here are a few details about it: GFS runs on Linux 2.2 kernels and allows Linux machines to share storage devices on a network. The network medium can be parallel SCSI, Fibre Channel, or whatever. GFS machines maintain locks around filesystem metadata operations to insure that only one machine at a time is modifying metadata: the metadata itself is distributed and the locking designed to reduce overhead so that high scalability (adding more clients) is possible -- we've achieved good performance with 8 machines connected across a FIbre Channel fabric to 8 JBODs with 4 Seagate FC drives each. We think our scalability is even higher, but don't have the hardware just yet to test out that theory :-) Distributed journaling (i.e., if one machine in the GFS SAN cluster fails other machine recover its journal to keep the file system metadata consistent) is now being implemented in GFS, and we expect to have that ready by early 2000. So what do shared disks give you? Better availability since if one machine fails the others can pick up the load. You can add disks, machines, or additional Fibre Channel switches to scale up your system. GFS includes a volume manager called pool that helps organize the network disks, and we are working on a mirroring implementation for this volume driver. We are also working on on-line resizing so that as you add more hardware to your SAN the Linux machines can simply slurp it into their existing file system organizations transparently. Finally, it gives you local disk performance with all the advantages of sharing and accessibility that network file systems provide. If this sounds interesting, check out our web site at http://www.globalfilesystem.org Matthew O'Keefe The GFS Group