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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 MD1. You're 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 begging for 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 yourself some 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 NOT cheaply. 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 another, bigger 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 this monkeying around to try to extract some last value from drives totally ill-suited for 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 www.newegg.com. They are specifically designed for RAID. Buy 4 and save yourself this massive 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 virtual *certainty* you're going to lose data if you go this route. Pay now, or pay later. What's your data worth?

<<RON>>

----- 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 having RAID5
(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 particulars: (Note, rounded to nearest GB)

80 GB /dev/hda 60 GB /dev/hdb 40 GB /dev/hdc 45 GB
/dev/hdd

GB /dev/md0 (RAID0)
40 /dev/hdc
15 /dev/hda1
55

/dev/md1 (RAID0)
45 /dev/hdd
10 /dev/hda2
55

/dev/md2 (RAID5)
55 /dev/md0
55 /dev/md1
55 /dev/hda3
55 /dev/hdb
220

Yea, OK, so like the 220 is a bit optimistic, but should get pretty close to
that.


What do you think?


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