[Dovecot] OT - small hd recommendation

Thomas Harold thomas-lists at nybeta.com
Wed Aug 31 22:41:19 EEST 2011

On 8/30/2011 5:43 PM, Daniel L. Miller wrote:
> A little OT - but I've seen a few opinions voiced here by various admins
> and I'd like to benefit.

RAID-10 is fine (note that the default mdadm RAID10 isn't actually 
RAID10, but it works well enough).  RAID-6 won't be faster (and will 
probably be worse) although RAID-6 does do a bit better in a 
double-drive failure over RAID-10.  The only way to get more performance 
out of (4) drives is to switch to 10k or 15k SAS (or SSDs).

For more information - see the Linux RAID mailing list:

One problematic issue with consumer-grade SATA drives (which may or may 
not bite you) is that they will not time out on errors fast enough to 
keep mdadm happy.  The "enterprise" grade drives are better about this 
(such as the ES.2 series), but for smaller arrays (6 drives or less) 
it's not as big of a deal.  For bigger arrays, it's a definite issue, 
especially if you try and do RAID-6 over 8+ drives.

If you're getting SMART errors, then it's time to swap the drives out. 
If mdadm is reporting sync errors or dropping drives from the array, 
then get your backups squared away ASAP before fiddling.

My knee-jerk reaction when I hear 4-drive RAID-10 is that it has no 
hot-spare.  Which means that as soon as 1 drive fails you're in 
dangerous territory (make sure it pages you automatically) since the 
array can't automatically repair.  Make sure you can properly identify 
the drive that fails (via the serial numbers) and don't try a hot-swap.

(Take a look at /dev/disk/by-id, /dev/disk/by-uuid, etc. Export a copy 
of that information on a daily/weekly basis off of the machine.  In a 
software RAID environment, it gives you better information about which 
drive serial # failed rather then relying on lights.)

Our mail server is 3-way RAID1 (triple mirror) for the OS and mail queue 
with a 5-disk RAID-10 (4+spare) for mail storage.

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