On Thu, 30 Dec 2010 09:23:56 +0100 Rogier Wolff <R.E.Wolff@xxxxxxxxxxxx> wrote: > > Hi, > > A friend has a webserver. He has 4 drive bays and due to previous > problems he's not content to have 3 or 4 drives in a raid5 > configuration, but he wants a "hot spare" so that when it takes him a > week to find a new drive and some time to drive to the hosting > company, he isn't susceptible to a second drive crashing in the > meantime. > > So in principle he'll build a 3-drive RAID5 with a hot spare.... > > Now we've been told that raid5 performs badly for the workload that is > expected. It would be much better to run the system in RAID10. However > if he'd switch to RAID10, after a single drive failure he has a window > of about a week where he has a 33% chance of a second drive failure > being "fatal". > > So I was thinking.... He's resigned himself to a configuration where > he pays for 4x the disk space and only gets 2x the available space. > > So he could run his array in RAID10 mode, however when a drive fails, > a fallback to raid5 would be in order. In this case, after the resync > a single-drive-failure tolerance is again obtained. > > In practise scaling down to raid5 is not easy/possible. RAID4 however > should be doable. > > In fact this can almost be implemented entirely in userspace. Just > remove the mirror drive from the underlying raid0, and reinitialize as > raid4. If you do this correctly the data will still be there.... > > Although doing this with an active filesystem running on these drives > is probably impossible due to "device is in use" error messages.... > > So: Has anybody tried this before? > Can this be implemented without kernel support? > Anybody feel like implementing this? > > Roger. > The kernel already supports this, though only with very recent kernels. I'm not 100% sure about mdadm support, but if it isn't there yet, it probably will be soon. You can convert a RAID10 to a RAID0. You probably have to remove two devices first, so there are just two working devices - no redundancy. mdadm --grow /dev/md0 --level=0 Then you can convert the RAID0 to RAID4 mdadm --grow /dev/md0 --level=4 Then add the good device back in mdadm /dev/md0 --add /dev/sdXX This should all work, though you should certainly test it before you depend on it at all. 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
