On Fri, Jul 17, 2009 at 11:49 AM, H. Peter Anvin<hpa@xxxxxxxxx> wrote: > Ric Wheeler wrote: >>> >>> The bottom line is pretty much this: the cost of changing the encoding >>> would appear to outweigh the benefit. I'm not trying to claim the Linux >>> RAID-6 implementation is optimal, but it is simple and appears to be >>> fast enough that the math isn't the bottleneck. >> >> Cost? Thank about how to get free grad student hours testing out things >> that you might or might not want to leverage on down the road :-) >> > > Cost, yes, of changing an on-disk format. In the context of BTRFS the only costs are development of the new erasure code and integration as well as the possible cost of carrying around two erasure codes (one for MD backwards compatibility, one for BTRFS). The availability of arbitrary M of N erasure codes is more attractive for BTRFS than MD because of the increased granularity. I.e. with BRTFS you might want a smaller stripe width because some sub-volume spans fewer spindles, or increased parity blocks you need increased redundancy for some small set of files. On the other hand— for performance reasons if you want additional redundancy beyond 'RAID6' you might just be better off writing more duplicates of the data blocks instead of additional RS syndromes since while the additional data blocks are not optimal from an error recovery perspective they will provide a performance gain of being able to spread reads out across additional disks. -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
