Kevin, On Mon, Jul 15, 2002 at 01:56:54PM -0500, Kevin Corry wrote: > In the current design, there are two cases when the COW table is written to > disk. Either when current COW sector is full, or on a clean system shutdown. > All snapshots will thus be persistent across a clean shutdown or reboot. > Currently, an async snapshot will be disabled if the system crashes. This > wasn't a big secret. In the latest HOWTO, the section on snapshotting > explains this, and in the EVMS gui, there is a note attached to the "async" > option saying this as well. > > We designed async snapshots in EVMS for maximum performance. Allowing for the > one above condition provides for a significant performance increase. Writing > the COW table only when it's full prevents a lot of unnecessary disk head > seeking. ... > But, the synchronous option is still available > for those who are scared about system crashes. Personally, I'm not that > scared. I'd have a hard time remembering the last time one of my production > machines crashed unexpectedly. So you are saying that your async snapshots should only be used on production machines, and where the data stored on the snapshot is so unimportant that you don't mind loosing it. Nice. In future could you mention this caveat when you post comparison benchmarks. device-mapper *does* ensure that the snapshot is always consistent. I don't believe the benchmarks posted at the top of this thread at all. Not only are you claiming poor performance for device-mapper, but that this performance degrades as chunk size reduces. device-mapper has been tested by a variety of people on many different machines/architectures and we've only ever seen a flat performance profile as chunk size increases, if anything, there is a very slight degradation as chunk size gets too large. For instance I just ran a test on my dev. box, this should not be considered a tuned benchmark by any means. dbench -2 on a 32M RAM system: no snapshot 8.22 8k 13.59 16k 13.99 32k 13.33 64k 12.90 128k 13.442 256k 13.654 512k 13.84 As far as I'm concerned you should be comparing this with the slower but consistent synchronous snapshots in EVMS. - Joe _______________________________________________ linux-lvm mailing list linux-lvm@sistina.com http://lists.sistina.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://www.sistina.com/lvm/Pages/howto.html
