<<< SSAG::DISK$ARCH2:[NOTES$LIBRARY.SSAG]ASK_SSAG.NOTE;32767 >>> -< Ask the Storage Architecture Group >- ================================================================================ Note 6530.2 How mirror set redundancy is maintained under hardware mirr 2 of 4 SSAG::LARY "teach 10,000 stars how not to dance" 21 lines 1-APR-1997 00:39 -------------------------------------------------------------------------------- If I understand your diagram correctly, it is an example of a redundancy technique called "Chained Declustering" which is touted as superior to RAID 0+1. In your example, Chained Declustering divides each disk in half (simplest way is two big contiguous halves, but performance may be better if the halves are interleaved) and shadows the halves, but in a staggered fashion. The MTDL (Mean Time to Data Loss) is slightly lower than RAID 0+1 because the number of two-drive combinations whose failure can produce data loss is larger, but larger than RAID 5. The advantage of chained declustering over RAID 0+1 in your example is that when a disk fails, the two disks on either side each increase their read load by 50% instead of having the single surviving mirrorset member increase its read load by 100%. Obviously this advantage is only meaningful when you need the redundancy in RAID 0+1 to improve read throughput over plain RAID 0. You can use chained declustering on any set of 2 or more disks, although you get no advantage at all over mirroring when you use it on 2 disks. And, if I remember correctly, you can divide the disks into arbitrarily many chunks (not just 2) and spread the read load on a failure to all the other disks in the chained declustering set. When you do this, the MTDL is the same as RAID 5. As far as I know, we have no shipping product that implements this.