Re: [gentoo-user] Duplicate identical Hard Disk

[Mark Knecht]

On Fri, Apr 2, 2010 at 5:02 AM, Dan Cowsill <danthehat@×××××.com> wrote:
> On Thu, Apr 1, 2010 at 10:09 PM, Mark Knecht <markknecht@×××××.com> wrote:
>>
>> With only 2 disks I personally think you're on the right path. With 3
>> disks I'm personally planning on RAID1 using 3 copies.
>> ...
>> My comment about RAID was that I am learning the hard (alas expensive)
>> way that not all disks can actually do RAID, at least not Linux
>> software RAID, and really be usable.
>>
>>
>
> From what I understand of software RAID in linux, it works on block
> devices, not disks.  This means if some endeavoring soul was brave
> enough to RAID even partitions on a device, it would work as normal.
> Perhaps you mean that not all properly functioning disks can do RAID?
> What sort of trouble are you running into?
>
> I've successfully deployed both RAID1 and RAID5 on my home media
> server for quite some time now.  While the initial time investment in
> reading documentation was considerable, since that time I've had no
> cause for trouble.  I keep smartmontools looking at the array member
> disks and regularly read through monthly smart reports of my drives.
>
> Also, if you have three disks, why not go for RAID5?  It is much
> quicker and I believe you'll end up with more space.  It is a bit of a
> pain to get mdadm to convert your RAID1 to a RAID5, but it is doable.
>
> DC

Good questions:

1) Yes, you can RAID partitions of drives. That's what I'm doing. You
can look at the Gentoo RAID/LVM Install guide to see an example of
using RAID0 and RAID1 on a single drive.

http://www.gentoo.org/doc/en/gentoo-x86+raid+lvm2-quickinstall.xml

2) I'm certainly not suggesting RAID doesn't work. It's just not
working for me, either due to new motherboard hardware or due to the
drives themselves. I'm currently betting it's the drives. The
background info, without getting too deeply into it, is that if the
drive supports SMART and SMART is enabled, then when doing RAID you
need guaranteed Time Limited Error Recovery (TLER) to ensure (I think)
that SMART works doesn't get in the way of the drive responding in the
appropriate amount of time or else the drive will fall out of the RAID
array. Turns out the WD  (according to different mailing list and
forums I've been looking at) has removed TLER on almost all of their
Green drive and some/many/most of the Blue and Black series. They are
supporting this in the RE drives though of which I've obtained two.
They are smaller and more expensive, but built for RAID, so I'm going
to try them out next.

http://en.wikipedia.org/wiki/Time-Limited_Error_Recovery

3) As I understand the subject you are correct about size and speed,
but a 3-disk RAID5 array can stand 1 disk failing whereas a 3-disk
RAID1 array can stand 2 disks failing. For this app (MythTV and seldom
used backup server) I don't need speed and size isn't a huge issue so
I chose 3-disk RAID1. (Note that the HTPC case I'm using supports up
to 3 drives only.) Because multiple drives purchased at the same time
generally come from the same production lot there's an additional
danger that if one drive fails then one more (or all) could fail at
the same time so I'm protecting myself against that. Again, this is
very specific to my current needs which is really to back up another
machine which will be RAID0 as it needs more disk I/O speed to support
12 processor cores.

As always, I'm certainly interested in info and ideas on this subject,
most especially now when I'm buying and building.

Cheers,
Mark