[rescue] RAID (was Re: Insane drives)

[Robert Novak]

On Tue, 10 Jul 2001 sambo at charm.net wrote:

> "Joshua D. Boyd" wrote:
> 
> > Ooh what I'd could do with a 90gig array.  What would that work out to
> > after doing a raid5 anyway?  How do you figure out before hand what raw
> > capacity you need to meet a specific raid5 capacity?
> 
> Raid 5 = 2/3 of total. Put in three 10G drives and you get a 20G array.

The second part is almost accurate, the first is wrong unless you only
have 3 disks in your raid set.

Safe assumptions are that you'll get 90% of the formatted space as
filesystem space (9.1GB should give you about 8.4GB usable filesystem
space--look at slice 2 on any sun drive for a good estimate) if you drop
minfree below 10%. I tend to use 1% myself, unless I know I have way more
space than I need.

RAID 5 will take one of the disks worth of capacity for parity storage. So
3 disks in a RAID 5 set give you 2 disks worth of capacity, or 16.8GB if
you have 3 9.1GB disks in your set. 

If you have 10 9.1GB disks in your set, you get 75.6GB or so (assuming no
spares). Might want to use a hotspare if you have the capacity (Sun's
Disksuite will let you create a hotspare pool, so put one disk in it and
attach it to your RAID 5 metadevice).

Note that software RAID 5 sucks. I think this has been hashed out here and
elsewhere several times in the past, but it bears saying again. I just do
RAID 0+1 when I'm using Disksuite--takes more disks (2x your needed space)
but is faster in software. If you can get a hardware RAID controller for
FC, you'd be in good shape (but probably somewhat poorer than when you
started).

I don't know if FC disks are any different in terms of how much truly
usable space you get--I'd guess they're no different since the platters
and stuff are the same between ST19171FC and ST19171WC most of the time?

-- 
Robert Novak, Indyramp Consulting * rnovak at indyramp.com * indyramp.com/~rnovak
        "And it's been a long December and there's reason to believe
           Maybe this year will be better than the last...." -- counting crows