On 30/12/2010 7:23 PM, Rogier Wolff 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.
Maybe I'm not quite understanding right, however you can easily do RAID6 with 4 drives. That will give you two redundant, effectively give you RAID5 if I drive fails, and save buttloads of messing around...
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