building a RAID systemo

Robert G. Brown rgb at phy.duke.edu
Wed Oct 8 21:45:08 EDT 2003


On Wed, 8 Oct 2003, Mark Hahn wrote:

> you should buy a single promise sata150 tx4 and four big sata disks
> (7200 RPM 3-year models, please).

I totally agree with everything Mark said and second this.  Although
3-year ata (lower) or scsi (higher) disks would be just fine too,
depending on how much you care to spend and how much it costs you if
things go down.

e.g. md raid under linux works marvelously well, and one can even create
a kickstart file so that it makes your raid for you on a fully automated
install, which is very cool.  It is also dirt cheap.  My home (switched
100 Mbps, 8-9 hosts/nodes depending on what is on) has a 150 GB RAID-5
server (3x80 GB 3-year ATA 7200 RPM disks) on a 2.2 GHz Celeron server
with an extra ATA controller so there is only one disk per channel.  It
cost about $800 total to build inside a full tower case with extra fans
including one with leds in front so that it glows blue.  You couldn't
get the CASE of a HW raid for that price, I don't think (although I
admit that it won't do hot swap and dual power supplies).

The total RAID/NFS load since 9/19 is:

root        11  0.0  0.0     0    0 ?        SW   Sep19   0:00 [mdrecoveryd]
root        21  0.0  0.0     0    0 ?        SW   Sep19   0:00 [raid1d]
root        22  0.0  0.0     0    0 ?        SW   Sep19   0:02 [raid5d]
root        23  0.0  0.0     0    0 ?        SW   Sep19   5:03 [raid5d]
...
root      4928  0.0  0.0     0    0 ?        SW   Sep19   2:58 [nfsd]
root      4929  0.0  0.0     0    0 ?        SW   Sep19   2:57 [nfsd]
root      4930  0.0  0.0     0    0 ?        SW   Sep19   3:00 [nfsd]
root      4931  0.0  0.0     0    0 ?        SW   Sep19   2:43 [nfsd]
root      4932  0.0  0.0     0    0 ?        SW   Sep19   3:00 [nfsd]
root      4933  0.0  0.0     0    0 ?        SW   Sep19   2:43 [nfsd]
root      4934  0.0  0.0     0    0 ?        SW   Sep19   2:56 [nfsd]
root      4935  0.0  0.0     0    0 ?        SW   Sep19   2:58 [nfsd]

(or less than 30 minutes of total CPU).  At 1440 min/day, for 18 days
(conservatively) that is about 0.1% load, on average.

This is a home network load, sure (which includes gaming and a fair bit
of data access, but no, we're not talking GB per day moving over the
lines).  In a more data-intensive environment this would increase, but
there is a lot of head room.  The point is that a 2.2 GHz system has a
LOT of horsepower.  We used to run entire departments of twenty or
thirty workstations using $10-20,000 Sun servers at maybe 5 MEGAHertz on
10 Mbps thinwire networks with fair to middling satisfaction.  My $800
home server has several thousand times the raw speed, about a thousand
times the memory, a thousand times the disk, AND it is RAID 5 disk at
that.  The network has only increased in speed by a factor of maybe
10-20 (allowing for switched vs hub).  Mucho headroom indeed.

BTW, our current department primary server is a 1 GHz PIII, although
we're adding a second CPU shortly as load dictates.  And if you are
planning your server to handle something other than a small cluster or
LAN where downtime isn't too "expensive" you may want to look at higher
quality (rackmount) servers and disk arrays in enclosures that permit
e.g. hot swap and that have redundant power.

   rgb

-- 
Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu



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