Bob Friesenhahn wrote: > On Wed, 15 Oct 2008, Tomas Ögren wrote: >>> ZFS does not support RAID0 (simple striping). >> >> zpool create mypool disk1 disk2 disk3 >> >> Sure it does. > > This is load-share, not RAID0. Also, to answer the other fellow, > since ZFS does not support RAID0, it also does not support RAID 1+0 > (10). :-)
Technically correct. But beware of operational definitions. From the SNIA Dictionary, http://www.snia.org/education/dictionary RAID Level 0 [Storage System] Synonym for data striping. RAID Level 1 [Storage System] Synonym for mirroring. RAID Level 10 not defined at SNIA, but generally agreed to be data stripes of mirrors. Data Striping [Storage System] A disk array data mapping technique in which fixed-length sequences of virtual disk data addresses are mapped to sequences of member disk addresses in a regular rotating pattern. Disk striping is commonly called RAID Level 0 or RAID 0 because of its similarity to common RAID data mapping techniques. It includes no redundancy, however, so strictly speaking, the appellation RAID is a misnomer. mirroring [Storage System] A configuration of storage in which two or more identical copies of data are maintained on separate media; also known as RAID Level 1, disk shadowing, real-time copy, and t1 copy. ZFS dynamic stripes are not restricted by fixed-length sequences, so they are not, technically data stripes by SNIA definition. ZFS mirrors do fit the SNIA definition of mirroring, though ZFS does so by logical address, not a physical block offset. You will often see people describe ZFS mirroring with multiple top-level vdevs as RAID-1+0 (or 10), because that is a well-known thing. But if you see this in any of the official documentation, then please file a bug. > > With RAID0 and 8 drives in a stripe, if you send a 128K block of data, > it gets split up into eight chunks, with a chunk written to each > drive. With ZFS's load share, that 128K block of data only gets > written to one of the eight drives and no striping takes place. The > next write is highly likely to go to a different drive. Load share > seems somewhat similar to RAID0 but it is easy to see that it is not > by looking at the drive LEDs on an drive array while writes are taking > place. ZFS allocates data in slabs. By default, the slabs are 1 MByte each. So a vdev is divided into a collection of slabs and when ZFS fills a slab, it moves onto another. With a dynamic stripe, the next slab may be on a different vdev, depending on how much free space is available. So you may see many I/Os hitting one disk, just because they happen to be allocated on the same vdev, perhaps in the same slab. -- richard _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
