On Friday February 13, davidsen@xxxxxxx wrote: > Julian Cowley wrote: > And in this case locking the bard door after the horse has left is > probably a path of least confusion. > > > Perhaps instead the documentation in mdadm(8) and md(4) could be > > updated to mention that raid10 is a combination of the concepts in > > RAID 1 and RAID 0, but is generalized enough so that it can be done > > with just two drives at a minimum. That would have caught my eye, at > > least. > > Good idea. Patches gladly accepted. > > Ob. plug for raid5E: the advantages of raid5E are two-fold. The most > obvious is that head motion is spread over N+2 drives (N being number of > data drives) which improves performance quite a bit in the common small > business case of 4-5 drive setups. It also puts some use on each drive, > so you don't suddenly start using a drive which may have been spun down > for a month, may have developed issues since SMART was last run, etc. > Are you thinking of raid5e, where all the spare space is at the end of the devices, or raid5ee where it is more evenly distributed? So raid5e is just a normal raid5 where you don't use all of the space. When a failure happens, you reshape to n-1 drives, thus absorbing the space. raid5ee is much like raid6, but you don't read or write the Q block. If you lose a drive, you rebuild it in the space were the Q block lives. So would you just use raid6 normally and transition to a contorted raid5 on device failure? Or would you really want to leave those blocks fallow? I guess I could implement that by using 8bits in the 'layout' number to indicate which device in the array is 'failed', and run a reshape pass that changes the layout, being careful not to re-write blocks that hadn't changed.... Not impossible, but I would much rather someone else wrote (and tested) the code while I watched... > While the distributed spare idea could be extended to raid6 and raid10, > the mapping gets complex. Since Neil is currently adding code to allow > for orders other than sequential in raid6, being able to quickly deploy > the spare on a once-per-stripe basis might at least get him to rethink > the concept. I think raid6e is trivial and raid6ee would be quite straight forward. For raid10, if you used a far=3 layout but only use the first two copies, you would effectively have raid10e. If you used a near=3 layout but only used 2 copies, you would have something like a raid10ee, but if you have 3 or 6 drives, all the spare space would be on the 1 (or 2) device(s). NeilBrown -- 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
