On Tue, Mar 3, 2009 at 1:08 PM, Mag Gam <[email protected]> wrote: > I was wondering if HAMMER will ever have network based RAID 5. After > researching several file systems it seems HAMMER is probably the > closest to achieve this problem and will make HAMMER a pioneer.
Intuitively I highly doubt network RAID5 is worth it. Even local disk RAID5 is unusable for many work loads. In contrast, check out some of the more flexible RAID10 modes available in Linux: http://en.wikipedia.org/wiki/Non-standard_RAID_levels#Linux_MD_RAID_10 You can get N/M effective space (N raw storage / M copies) with RAID0-like striping for all of it. It performs very well and certainly much better than the parity-based RAID5. Imagine how RAID5 would work with network devices: Read old data block from one server Read parity block from another server Generate new parity block Write data block to one server Write parity block to another server All with NO atomicity guarantees, so HAMMER would have to pick up the slack. Even in the best case you have 8x the latency of a single trip to a machine (4 request/response pairs of 2 IOs each). All compared to a one round trip (2 IOs) to write to a plain slave, or N round trips for N redundant copies. What is an acceptable penalty on local disks is pretty heavy for network storage. If you really want, you can use vinum over iSCSI to get networked RAID5, but it will not perform well. -- Dmitri Nikulin Centre for Synchrotron Science Monash University Victoria 3800, Australia
