[NTLUG:Discuss] Dell Server with PERC RAID

[Chris Cox]

Neil Aggarwal wrote:
> Chris:
> 
> I looked at the referenced articles and I don't see what the 
> difference is between RAID 0+1 and RAID 10.

Uhh... the LSI pictures do a pretty good job of
showing what is going on (you know.. picture = 100 words).
I'm not sure what I could say to "clarify" it more...

I'll give it my best shot though..

In RAID 10 pretend that you have 3 DRIVES.  Now in
reality each DRIVE is really a number of drives... so
let's say DRIVE = 2 drives

The 2 drives in each DRIVE mirror their
contents across each other.

Now apply striping across the 3 DRIVES. When
determining size on a 10 it's common to only
have 2 drives in each DRIVE and you increase
the number DRIVES to increase the size of
the array.  You can have DRIVES that have
more than 2 drives in them if you want (a 3
disk mirror).. but there's a cost<->reliability
equation to be considered.  RAID10 as mentioned
is "expensive" (though not like it used to be..
it's practical nowadays)... using a 3 way mirror
set just adds to the cost (while increasing
reliability) without increasing amount of space
available.

I can afford to lose more than one drive in
a RAID10.  Remember that each DRIVE consists
of two mirrored drives... so each stripe
can afford a disk failure.  If you use a 3-way
mirror, obviously you could theoretically
achieve super-duper reliability, etc.

(realize that RAID 0+1 and RAID 10 are not really
meant to be grown.. this is something you design
ahead of time)

In RAID 0+1 pretend that you have 2 DRIVES.  Now in
reality each DRIVE is really a number of drives... so
let's say DRIVE = 2 drives (same as before)

The 2 drives in each DRIVE stripe their data
across each other.

Now apply mirroring across the 2 DRIVES.  When
determining size on a 0+1 you usually don't see more than
2 DRIVES.. instead you consider the number of drives
in each DRIVE.

Just like RAID1, I can't handle more than one
failure in RAID0+1.  One failure in one of my
DRIVEs kills that DRIVE (invalidates the whole stripe
set).  This tends to invalidate more drives than
with RAID10, even if you add more DRIVES.  Adding
more DRIVES wastes far more disk than with RAID10
and doesn't give you as high a reliability gain.

Clear as mud?  I like the pretty pictures better.



> 
> Can you please clarify?
> 
> Thanks,
> 	Neil
> 
> 
> --
> Neil Aggarwal, JAMM Consulting, (972)612-6056, www.JAMMConsulting.com
> FREE! Valuable info on how your business can reduce operating costs by 
> 17% or more in 6 months or less! => http://newsletter.JAMMConsulting.com
> 
> 
>>-----Original Message-----
>>From: discuss-bounces at ntlug.org 
>>[mailto:discuss-bounces at ntlug.org] On Behalf Of Chris Cox
>>Sent: Friday, September 26, 2003 10:08 AM
>>To: NTLUG Discussion List
>>Subject: Re: [NTLUG:Discuss] Dell Server with PERC RAID
>>
>>
>>Cameron, Thomas wrote:
>>
>>>RAID 0+1 (often called RAID 10) is two RAID 0 arrays which are
>>
>> > mirrored (RAID 1).  Gives you the benefit of fast striping
>> > plus the fault tolerance of mirroring.  It's just ghastly 
>>expensive.
>>
>>Actually RAID0+1 and RAID10 are radically different things.
>>http://www.acnc.com/04_01_10.html
>>http://www.lsilogic.com/products/stor_prod/raid/backgrounder1.html
>>
>>Low end RAID controllers do 0+1 (it's easier to handle).  RAID 10 is
>>a highly reliable RAID, RAID 0+1 isn't.  You'll only find RAID 10
>>on your high end controllers (e.g. a MegaRAID Elite 1600 can do
>>RAID 10, perhaps a bit old, but works well with Linux).
>>
>>Since disk has become cheap, creation of RAID 10's is now
>>quite practical (remember 4G drives used to cost $3000).
>>On systems where it works right you get the performance of
>>RAID0 with the reliability of RAID1 AND.. unlike RAID0+1,
>>you can more than one drive failure (depends on location and
>>number of columns though).
>>
>>RAID10 is considered the creme de la creme of RAID (but
>>probably the most expensive RAID config... but practical
>>considering today's disk prices).  You may have to pay
>>to get a controller and setup that supports RAID10 though.
>>
>>If you need HIGH reliability and FAST reads/writes...
>>RAID10 might be a viable choice for you.  Most roll
>>the dice and use good quality SCSI and run RAID0+1
>>though (mostly because of limitations in their choice
>>of RAID controller though).  Or run just RAID1 or
>>run RAID5 (slow writes and slow rebuilds).
>>
>>The web links above do a pretty good job showing
>>the differences between the various RAID levels.
>>
>>
>>
>>
>>
>>
>>
>>
>>
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