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In-Reply-To: <201104100850.41768.peter@humphrey.ukfsn.org>
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Date: Sun, 10 Apr 2011 06:50:59 -0700
Message-ID: <BANLkTikZRZCsPD84mgpoNOMFirE_osFJ9Q@mail.gmail.com>
Subject: Re: [gentoo-user] [OT] Disk recommendations?
From: Mark Knecht <markknecht@gmail.com>
To: gentoo-user@lists.gentoo.org
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On Sun, Apr 10, 2011 at 12:50 AM, Peter Humphrey
<peter@humphrey.ukfsn.org> wrote:
> On Saturday 09 April 2011 22:01:18 Mark Knecht wrote:
>
>> Are you running a RAID?
>
> Yes; mdadm RAID-1, with LVM on top, as in the Gentoo how-to:
> http://www.gentoo.org/doc/en/gentoo-x86+raid+lvm2-quickinstall.xml
>
>> Are you looking for a little redundancy or a lot of redundancy?
>
> I'm just speculating at the moment, from a dabbler's point of view; what benefits
> would accrue from switching from RAID-1 to RAID-5 or above? And, in particular,
> what are the comparative virtues of the Samsung disks?
>
My understanding is there's nothing more reliable than RAID1. mdadm
allows N-wide RAID1. My RAID1's are currently 3-drive.
Typically the higher RAID numbers are for trading off storage space,
redundancy and in some cases throughput. My 5-drive RAID6 gives me
(again, my understanding) equivalent redundancy to a 3-drive RAID1. I
can lose 2 drives in either RAID before I risk losing everything with
a 3rd drive failure, but I only get the storage of 3-drives. A 5-drive
RAID5 would lose everything with 2 drive failures but gets 4 drives
of storage.
As for Samsung drives I have no experience. However one common problem
I read about again and again is a RAID user who loses 1 drive and
then, while in the process of fixing the RAID, loses a second drive.
Most of us (myself included) buy identical drives all at the same time
from the same vendor. This means all the drives were likely from the
same manufacturing batch and, if they are drives that will fail at all
then the group will likely experience multiple drive failures. The
underlying idea of RAID is that the drives are not likely to fail at
the same time giving us time to fix the array. However, if /dev/sda
fails the chances of /dev/sdb failing is higher if they were built at
the same time in the same plant.
Reading the mdadm list for the last couple of years it seems that many
folks running data centers intentionally buy drives from multiple
manufactures, or drives of different sizes from the same manufacturer,
hoping to lower the chances of multiple failures at the same time.
What I did myself was buy 5 drives initially, 3 from Amazon, 2 from
NewEgg. For spares I then waited 2 months, bought one more drive, and
waited another 2 months and got one more. In my case all my drives are
WD RAID Edition drives which have higher reliability specs than the
commercial drives. (And are more expensive and smaller)
As for hardware RAID the risk I hear about there is that if the
controller itself fails then you need an identical backup controller
or you risk the possibility that you won't be able to recover
anything. I don't know how true that is or whether it's just FUD.
Cheers,
Mark