(Subject line amended by me :-) On 12/04/2011 17:56, Mathias BurÃn wrote: [...]
I'm approaching over 6.5TB of data, and with an array this large I'd like to migrate to RAID6 for a bit more safety. I'm just checking if I understand this correctly, this is how to do it: * Add a HDD to the array as a hot spare: mdadm --manage /dev/md0 --add /dev/sdh1 * Migrate the array to RAID6: mdadm --grow /dev/md0 --raid-devices 7 --level 6
You will need a --backup-file to do this, on another device. Since you are keeping the same number of data discs before and after the reshape, the backup file will be needed throughout the reshape, so the reshape will take perhaps twice as long as a grow or shrink. If your backup-file is on the same disc(s) as md0 is (e.g. on another partition or array made up of other partitions on the same disc(s)), it will take way longer (gazillions of seeks), so I'd recommend a separate drive or if you have one a small SSD for the backup file.
Doing the above with --layout=preserve will save you doing the reshape so you won't need the backup file, but there will still be an initial sync of the Q parity, and the layout will be RAID4-alike with all the Q parity on one drive so it's possible its performance will be RAID4-alike too i.e. small writes never faster than the parity drive. Having said that, streamed writes can still potentially go as fast as your 5 data discs, as per your RAID5. In practice, I'd be surprised if it was faster than about twice the speed of a single drive (the same as your current RAID5), and as Neil Brown notes in his reply, RAID6 doesn't currently have the read-modify-write optimisation for small writes so small write performance is liable to be even poorer than your RAID5 in either layout.
You will never lose any redundancy in either of the above, but you won't gain RAID6 double redundancy until the reshape (or Q-drive sync with --layout=preserve) has completed - just the same as if you were replacing a dead drive in an existing RAID6.
Hope the above helps! Cheers, John. -- 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
