From: linux-lvm-bounces redhat com
[mailto:linux-lvm-bounces redhat com] On Behalf Of Ron Watkins
Sent: Tuesday, March 01, 2005 11:10 PM
To: Erik echohome org; LVM general discussion and development
Subject: Re: [linux-lvm] Building up a RAID5 LVM home server (long)
This is absolutely insane. It is among the dumbest designs
I've ever seen.
You are using HDA as a component in MD0 and MD1, and then
using HDA AGAIN as
part of MD2 directly, while using it indirectly via MD0 and
going to A) bottleneck on HDA, B) you're going to beat that
drive to death,
and C) if that drive goes, you are HOSED. Plus, you are just
problems with potential bugs in the RAID driver code. This
whole setup is
an INCREDIBLY bad idea. You're trying to 'be clever' to save
money, and all you're doing is buying trouble.
The way RAID5 is meant to work is with disks of approximately
the same size.
RAID5 is not expandable, unless you have a very expensive hardware
controller. There are algorithms that will let you expand
the size of a
RAID5 volume, but they have not, to my knowledge, been
implemented in open
source. You CANNOT do what you want to do, cheaply. You
can spend a great
deal of money to satisfy most of your design parameters, but
If you want it cheap, use fixed drives of about the same
size, and don't
think about expansion. When you're ready to expand, hook up
RAID and copy your data. In NO case can you use that
hodgepodge of junk
drives you've collected.
Most of your drives are obsolete. Keep the biggest one, buy
at least two
more of the same size, and set up a RAID5 using that. All
around to try to extract some last value from drives totally
the purpose is going to cost you far, far more than new
drives ever could.
Hell, keep the smaller ones around, put them into a concatenated LVM2
volume, and use them as a backup. It's not the best backup
in the world,
but it's better than nothing.
Do it right. This is your data you're trying to save. You
can get very
nice 250-gig PATA Western Digital drives for $165 from
are specifically designed for RAID. Buy 4 and save yourself
headache. If you don't need that much space, buy smaller drives.
Or, you can persist in trying to be clever, but it's a
you're going to lose data if you go this route. Pay now, or
What's your data worth?
----- Original Message -----
From: "Erik Ohrnberger" <erik echohome org>
To: "'LVM general discussion and development'" <linux-lvm redhat com>
Sent: Tuesday, March 01, 2005 10:27 PM
Subject: RE: [linux-lvm] Building up a RAID5 LVM home server (long)
> ... SNIP ...
>> >What if the I broke everything into 10 GB pieces, and
>> created multiple
>> >raid5 sets? Then I could LVM2 them together and have a large
>> >filesystem that way.
>> >a=20GB, b=30GB, c=40GB
>> >a-1 + b-1 + c-1 = md0 (approx 30 GB storage)
>> >a-2 + b-2 + c-2 = md1 (approx 30 GB storage)
>> > b-3 + c-3 = md2 (waiting for one more drive)
>> > c-4 = md3 (waiting for two more drives)
>> This is sorta what I do. But in my opinion the gain of
>> (over RAID1) is when you get over 3 disks... at 3 disks you are
>> burning 33% for redudnacy... 25% or 20% or 17% sounds
better to me.
>> I guess if
>> you go too far it costs in calculating the parity.
> Overhead: Yea, OK. Nothing is without a price.
> I fooled around with various ideas, and came up with this for my
> (Note, rounded to nearest GB)
> 80 GB /dev/hda 60 GB /dev/hdb 40 GB /dev/hdc 45 GB
> GB /dev/md0 (RAID0)
> 40 /dev/hdc
> 15 /dev/hda1
> /dev/md1 (RAID0)
> 45 /dev/hdd
> 10 /dev/hda2
> /dev/md2 (RAID5)
> 55 /dev/md0
> 55 /dev/md1
> 55 /dev/hda3
> 55 /dev/hdb
> Yea, OK, so like the 220 is a bit optimistic, but should get pretty
> What do you think?
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