Reply to post: Comment from Forum member who has forgotten his password

Crowd-sourcing interpretation of IBM RAID 5 extension paper

Chris Mellor 1

Comment from Forum member who has forgotten his password

Basically the paper gives the theoretical proof and underpinnings for SD codes. SD codes are more efficient than RAID-6 in that you do not have to dedicate 2 disks for parity which tolerate the failure of one disk and one sector. ie RAID-6 protects you from a disk failure and an URE (unrecoverable read error) during a RAID rebuild.

With SD codes, a disk and a sector within a RAID stripe are dedicated for parity which is more efficient and actually maps to how devices fail. ie Entire disks rarely fail, what is more likely is the failure of a sector within the device. The USENIX paper quoted in the comments has more information. A key quote is below

"We name the codes “SD” for “Sector-Disk” erasure codes. They have a general design, where a system composed of n disks dedicates m disks and s sectors per stripe to coding. The remaining sectors are dedicated to data.

The codes are designed so that the simultaneous failures of any m disks and any s sectors per stripe may be tolerated without data loss"

The research paper offers the proof for why this is so.....

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Chris comment - perhaps we need erasure codes to recover forgotten passwords....

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