[TriLUG] general RAID questions

[Joseph Mack NA3T]

I've just worked through the RAID section of Alan Porter's 
Jan presentation this year.

http://trilug.org/~porter/meetings/2011-01-13_AlanPorter_raid-lvm-luks.odp

I've not done any RAID and have these questions.

o is software raid the default in linux?

with hardware RAID a light on a failed drive comes on. 
Presumably the hardware driver card knows that the disk has 
failed a r/w, fails out the disk, runs in degraded mode and 
turns on the light. You hotswap the drive (presumably the 
failure is visible with software too, but I assume some poor 
flunkey patrols the racks once a day with a bucket of 
replacement disks). However with software raid, there won't 
be lights.

o What does a disk failure (eg click of death) look like on 
s/w raid? If a disk gets a r/w fail does mdadm know about it 
straight away? Do you check the output of mdadm every 5 mins 
with Nagios/rrdtool type scripts and send messages to 
someone to change the disk?

o with s/w raid, if you get a notice that /dev/sdm1 has 
failed, how do you find drive /dev/sdm in a rack if there's 
no light to turn on? Do you have to label all your drive 
bays at install time?

o can you run

smartctl -t long /dev/sdm

on each of the RAID devices? (I assume multipath complicates 
things here. Do you test each branch of the multipath 
separately?)

o s/w raid requires intervention at the keyboard to remove 
the old drive from mdadm, go to the racks to hotswap the 
drive, return to the keyboard, add the new drive, and check 
that the rebuild suceeds. If you have 100s of racks, I 
assume this process doesn't scale. So wouldn't everyone be 
running hardware RAID?


Thanks Joe

-- 
Joseph Mack NA3T EME(B,D), FM05lw North Carolina
jmack (at) wm7d (dot) net - azimuthal equidistant map
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