[K12OSN] Re: RAID 10

[Owen O Donovan]

(Aside) Thanks for the reference to the  material re rocketraid.  I'm
interested is setting up a bunch of NAS boxes and was looking at that as an
option.

The following are some notes about raiding focused around smaller terminal
server systems (30 - 60) users on 2 to 4 drives.   It seems to me that going
beyond that number is where you want to pull out the storage component and
make it independent of the terminal server hardware.

In addition to the redundancy benefit of a mirror (RAID1) which Jim spoke to,
the read access behaves just like striping. So a 2 drive mirrored system is
capable of supporting many more users than a single drive. Since common usage
patterns are  generally "attribute intensive" (listing directories, getting
file sizes etc)and reading smaller files, the net impact of using a simple
mirror is substantial. Stats from hdparm bear this out

Applications and usage patterns make a difference. On a small box, RAID1 holds
its own for home directories because most accesses are attribute reads. As the
size of your server goes up, RAID 5 is good economical home directory
solution: Your added cost for redundancy is only 1 additional drive/partition
(or 2 drives/partitions if you want hot spares). 

Software/partition-based RAID has some real advantages in smaller systems:
Some items are best left unRAIDed.  Swap is better on separate partitions
on multiple drives for several reasons.  The squid proxy cache prefers
unRAIDed partitions on multiple drives. So if you're doing lots of user apps
and cache/proxying your users traffic, using software RAID allows you to mix
and match partitions on multiple drives most economically.  The cost is a
small CPU overhead; the saving is an expensive and unnecessary RAID controller


RAID1 has another benefit sometimes used as an archiving mechanism. Since the
data is similar on both drives, one can be pulled and have all the data
available. A fresh drive can be installed and the mirror resync'd.  It's also
comforting to know that when the machine blows up, booting under Knoppix gives
you ready access to all the data on either drive in a mirror.

RAID0 trades off reliability for performance (where much of the performance
can be regained by a mirror with the added reliability). IMHO RAID0 has little
to offer for a general purpose server. RAID10 raises performance and
reliability but is the most expensive way of doing it and isn't really
appropriate for smaller installations.

oo



On Fri, 3 Dec 2004 14:43:22 -0600 (CST), Eric Smith wrote
> You also might want to consider Raid 10
> http://www.acnc.com/04_01_10.html
> 
> Highpoint makes an excellent card, the RocketRaid, tons of great features
> and a company with excellent Linux support (the 8 SATA port version
> SCREAMS)
> http://www.newegg.com/app/ViewProductDesc.asp?description=16-115-020&depa=0
> http://www.newegg.com/app/ViewProductDesc.asp?description=16-115-012&depa=0
> 
> > RAID5 is a very good way to protect your data due to the parity but when
> > using RAID5 you take pretty bit (up to 75%)  WRITE performance hit.
> > Only READ performace is gained when using RAID5.  Also depending on how
> > many drives you use with RAID5 you loose storage.  You need a minumum of
> > 3 drives to do RAID5
> >
> > RAID0 gives the best performance by far but it is not fault tolerant at
> > all.
> >
> > Here is a good site for some information on RAID.
> > http://www.acnc.com/04_00.html
> >
> > Jack
> 
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Owen O'Donovan
Battlefords School Division #118
(ph) (306) 937 7702
(fx) (306) 937 7721

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