On Tue, Jan 27, 2009 at 05:26:08AM +0100, 'Keld Jørn Simonsen' wrote: > On Mon, Jan 26, 2009 at 11:06:32AM -0500, GeneralNMX wrote: > > > > >From my understanding, there is fault tolerance and then there is the chance > > of a disk dying. Obviously, the more disks you have, the greater chance you > > have of a disk dying. If we assume all disks start out at some base chance > > to fail and degrade, putting multiple RAID types on the same disks can > > dramatically increase the wear & tear as the number of disks increase, > > especially when you have both a raid5 (which doesn't need to write to all > > disks, but will read from all disks) and a raid10 (which probably will write > > and read to all disks) on the same physical array of disks. Since fault > > tolerance is there to decrease the problems with disks dying, my setup is > > obviously sub-optimal. Whenever I access my RAID10, I'm also ever so > > slightly degrading my RAID5 and RAID1, and visa-versa. > > Your arrangement does not increase the wear and tear, as far as I can > tell. This compared to a solution where you only have one big raid10,f2 > raid. Actually your wear and tear would be lower, because raid5 does > not write so much if you mainly deal with bigger files, and not database > like operations. > Compared to raid10,f2, raid5 only writes 1/3 of the data for redundancyi in a 4-drive setup, and it does it in a striping manner, so raid5 is quite fast for sequential writing. > > Now, as for the I/O Wait, this happens when I try to access both the RAID10 > > and RAID5 at the same time, especially if I'm moving a lot of data from the > > RAID10 to the RAID5. > > I think this would be the same if you moved the data (copying it) within > the RAID10, or within the RAID5. Please try it out, and I would be > interested also to hear your results. Of cause moving around big files is IO bound. I think the theoretical best performance is sequential read time for the one raid, plus theoretical write time for the other raid, hoping that random read/write can be minimized. The theoretical read performance for raid10,f2 is almost 4 times nominal read speed, and theoretical write time for the raid5 is almost 3 times nominal speed, in your 4-drive setup. I tried some of it out with "cp", just on a single normal partititon, and it looks like "cp" minimizes the random read/write. I would be interested in hearing some performance fugures from you. Best regards keld -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
