Daniel Phillips - Sistina Software

Simple, Robust Software RAID for Linux 2.6

Paper (PDF) and Paper (Postscript) and Slides (OpenOffice) .

Linux's new Device Mapper subsystem provides efficient facilities for concatenating, striping and mirroring physical volumes into a single logical volume, but support of RAID level 5 is left to the existing Multiple Device subsystem. Though the Device Mapper and Multiple Device subsystems can be combined to work around this problem, it requires extra administration work, adds an extra level of processing overhead, and does not satisfy Device Mapper's original goal of simplicity.

A new RAID plug-in for Device Mapper is introduced here to provide RAID5-like functionality for physical volume configurations consisting of 2^n + 1 disks, where each logical block is split across 2^n disks, and parity information is written to the remaining disk. This strategy resembles the old RAID 3 strategy, and avoids one of the greatest sources of complexity in RAID 5, which is the need to read before writing in order to update parity information. This in turn removes the need for an extra block cache in the RAID driver. With the IO path thus simplified, performance for largely sequential write loads can approach the combined IO bandwith of the physical devices. Versus RAID 5, random IO loads generate higher seeking and lower rotational latency penalties, so random IO performance remains acceptable.