Re: Growing 6 HDD RAID5 to 7 HDD RAID6

[John Robinson <john.robinson@anonymous.org.uk>]

(Subject line amended by me :-)

On 12/04/2011 17:56, Mathias Burén 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@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html