[linux-lvm] Building up a RAID5 LVM home server (long)

Wed Mar 2 04:27:49 UTC 2005

Ron,
    Well, there is no mistaking this feedback  ;-)

    Thanks for the direct, succinct feedback.  I think you are right in your
points.

    The search goes on to balance flexibility, cost, and benefits, as
always.

    Erik.

> -----Original Message-----
> From: linux-lvm-bounces at redhat.com 
> [mailto:linux-lvm-bounces at redhat.com] On Behalf Of Ron Watkins
> Sent: Tuesday, March 01, 2005 11:10 PM
> To: Erik at 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 
> 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 at echohome.org>
> To: "'LVM general discussion and development'" <linux-lvm at 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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> > read the 
> LVM HOW-TO at 
> http://tldp.org/HOWTO/LVM-HOWTO/
> > 
> 
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