Software RAID using ZFS
ZFS offers software RAID through its
RAID-Z and mirroring organization schemes.
RAID-Z is a data/parity distribution scheme like
RAID-5, but uses dynamic stripe width: every block is its own RAID stripe, regardless of blocksize, resulting in every RAID-Z write being a full-stripe write. This, when combined with the copy-on-write transactional semantics of ZFS, eliminates the
write hole error. RAID-Z is also faster than traditional RAID 5 because it does not need to perform the usual
read-modify-write sequence.
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As all stripes are of different sizes, RAID-Z reconstruction has to traverse the filesystem metadata to determine the actual RAID-Z geometry. This would be impossible if the filesystem and the RAID array were separate products, whereas it becomes feasible when there is an integrated view of the logical and physical structure of the data. Going through the metadata means that ZFS can validate every block against its 256-bit checksum as it goes, whereas traditional RAID products usually cannot do this.
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In addition to handling whole-disk failures, RAID-Z can also detect and correct
silent data corruption, offering "self-healing data": when reading a RAID-Z block, ZFS compares it against its checksum, and if the data disks did not return the right answer, ZFS reads the parity and then figures out which disk returned bad data. Then, it repairs the damaged data and returns good data to the requestor.
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RAID-Z does not require any special hardware: it does not need NVRAM for reliability, and it does not need write buffering for good performance. With RAID-Z, ZFS provides fast, reliable storage using cheap, commodity disks.
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There are three different RAID-Z modes:
RAID-Z1 (similar to RAID 5, allows one disk to fail),
RAID-Z2 (similar to RAID 6, allows two disks to fail), and
RAID-Z3 (allows three disks to fail). The need for RAID-Z3 arose recently because RAID configurations with future disks (say, 6–10 TB) may take a long time to repair, the worst case being weeks. During those weeks, the rest of the disks in the RAID are stressed more because of the additional intensive repair process and might subsequently fail, too. By using RAID-Z3, the risk involved with disk replacement is reduced.
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Mirroring, the other ZFS RAID option, is essentially the same as RAID 1, allowing any number of disks to be mirrored.
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