On Tuesday 25 February 2003 09:57 pm, Joe Thornber wrote: > As far as I'm aware the only reason LVM doesn't have raid5 is that > nobody has written it. What is the serialisation problem ? Software RAID in 2.4 maintains a stripe cache that must be the same size as the incoming reads and writes. Usually this is 4k for standard filesystem io. But e.g. if you do a mkfs, then it can change. When md gets a request for a different sized read or write, it flushes the cache and rebuilds for the new size. That's a bad feature in LVM's case, because once you make a snapshot, you get a barrage of requests coming in at different sizes: filesystem (4096), snapshots (512) and I think syncs (0), so in effect there is no cache since it's syncing several times per second. Like I said, the perfomance is very bad. I oopsed a machine today by snapshotting an incoming FTP directory. Sorry, I sort of threw up my hands earlier. But tomorrow morning I'm going to see how repeatable it is and post to -dev. Thanks. -- Jason Smith Open Enterprise Systems Bangkok, Thailand _______________________________________________ linux-lvm mailing list linux-lvm@sistina.com http://lists.sistina.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
