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Richard Pieri <richard.pieri at gmail.com> writes: > On 4/11/2013 9:28 AM, Derek Atkins wrote: >> The reason I'm looking at a filesystem here is that the WAY writes occur >> can affect the write-holes you get in RAID5 and RAID6. For example, ZFS >> does not overwrite the existing block, it will write to a new block and >> then after the write succeeds will it change the block-pointer. > > COW does not prevent write holes. > > ZFS prevents write holes by enforcing atomicity of all writes to > storage. It does this by controlling all of the I/O caching involved in > the write process from system RAM down to the write acceleration cache > on the disks themselves. ZFS updates the file system only after all > cache points have confirmed being flushed. > > If any of these points lie about their status then write holes can > appear under power fault conditions. The RAID level does not matter. If > the hardware does not provide for the required write atomicity then you > can suffer write holes under power fault conditions. > > Both ZFS and Btrfs provide facilities for automatically "erasing" write > holes. The process is called "scrubbing". The scrubbing process walks > through the entire file system tree, recalculates all file and metadata > checksums, and compares them to the stored checksums. Errors are > repaired using replica data. Oracle's documentation recommends a weekly > scrubbing schedule for consumer-grade disks and a monthly scrubbing > schedule for server-grade disks. Fair enough... I don't know if standard (e.g. DM-level) RAID5 or RAID6 provide for said "scrubbing"? Or detecting/handling disk read or (or worse, disk write) failures. -derek -- Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory Member, MIT Student Information Processing Board (SIPB) URL: http://web.mit.edu/warlord/ PP-ASEL-IA N1NWH warlord at MIT.EDU PGP key available
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