2011-05-27 13:50, Frank Van Damme wrote:
Sequential? Let's suppose no spares. 4 mirrors of 2 = sustained bandwidth of 4 disks raidz2 with 8 disks = sustained bandwidth of 6 disks
Well, technically, for reads the mirrors might get parallelized to read different portions of data for separate users (processes). In case of ZFS that might *theoretically* be metadata vs. data blocks, if they were *theoretically* co-located in different parts of the platters. More realistically, if you have some other workload beside the one sequential read (which due to COW and/or dedup and following fragmentation would not likely be sequential really), this statement might be more realistic: "the 4*2 mirror may give you up to 8 disks of bandwidth (in reads)". And if the ZFS is supposedly smart enough to use request coalescing as to minimize mechanical seek times, then it might actually be possible that your disks would get "stuck" averagely serving requests from different parts of the platter, i.e. middle-inside and middle-outside and this might even be averagely more than 2x faster than a single drive (due to non-zero track-to-track seek times). This is purely my speculation, but now that I thought about it, can't get rid of the idea ;) ... -- +============================================================+ | | | Климов Евгений, Jim Klimov | | технический директор CTO | | ЗАО "ЦОС и ВТ" JSC COS&HT | | | | +7-903-7705859 (cellular) mailto:jimkli...@cos.ru | | CC:ad...@cos.ru,jimkli...@mail.ru | +============================================================+ | () ascii ribbon campaign - against html mail | | /\ - against microsoft attachments | +============================================================+ _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
