[WBEL-users] RAID 5 vs. RAID 10

[Kirby C. Bohling]

On Mon, Jul 25, 2005 at 05:56:17PM +0800, Andrew Vong wrote:
> 
> Hi,
> 
> I am looking into purchasing a new server. This server will be 
> mission-critical.
> 
> I have read and somewhat understood the theories behind RAIDs 0, 1, 5, 10 & 
> JBOD. However, I would like to get some feedback from those who have 
> experience in implementing and recovering from a HDD failure using RAID.
> 
> Hardware specs include:-
> 
> Dual Xeon 3.2 GHz
> 2 GB RAM
> 
> I would like to implement hardware RAID but am unsure as to which would be 
> most suitable for my needs. Any advice is appreciated.
> 
> Option 1 - RAID 5 (3 hdd's) + 1 hot spare
> 
> Option 2 - RAID 10 (4 hdd's) + 1 "cold" spare (in the shelf)
> 
> 

Everything I'm saying is about RAID5 on a software RAID
implementation.

> Questions I have :-
> 
> 1) When should RAID 5 be implemented?
> 2) When should RAID 10 be implemented?
Depends on a lot of factors.  There are cases for both.  Assuming
Money is no option, and the extra number of drives isn't a
heat/space/size/connectors factor, I'd always chose RAID10.

It's when those other things that become a problem that I consider
doing RAID5.

> 3) Is RAID 5 with a hot spare safer than RAID 10 with a "cold" spare?
Depends on the failure mode.  With RAID5 it's impossible to recover
from two drives failing at the same time.  With RAID 10, you have a
50-50 split.  The more drives, the more skewed it is towards RAID10
being a better failure.  After that it depends on why the drives
failed and how long it takes you to get a drive replaced.  All
failures where I work, generally take no more then an hour to
replace.  So the difference between hot and cold is that one hour
gap.

With RAID10 in a dire emergency you have two copies of everything.
So I'd feel much safer.

> 4) Is it possible to configure RAID 10 to have a hot spare?
Yes, if you use mdadm.  man mdadm search for "spare-group".

> 5) Should one of the HDDs fail, a hot spare w kick-in immediately and begin 
> rebuilding. As I am planning to put in 300 GB HDDs, how long would this 
> take on a RAID 5 vs. RAID 10?

On RAID5 a really, really long time.  I know that our 160Mbit/sec
(or is that Mbyte/sec) drives take 12-15hours with no other load.
However, that's a 14 disk RAID5.  In order to recover a RAID5 you
have to be reading from all of the other disks.  So you'll
completely saturate the I/O bus.  With a RAID10, you are limited to
the speed of the drives.  (I found this out the hardway on a 14disk
RAID5 array).  To rebuild a 12 disk RAID5 you have to read 11 disks,
and write to one.  With a RAID10, you read from one, and write to
one.

> 6) Will there be a degradation in performance for users on the system (RAID 
> 5 vs. RAID 10)?

RAID5 generally has a penalty on writes smaller then the block size.
Shrinking the blocksize of the RAID5 device, slows down the RAID5.
It's a pick you poison deal.

> 7) What are the disadvantages of using RAID 5 vs. RAID 10?

I only use RAID5 on very large filesystems that are low usage,
archival.  Where the costs of implementing RAID10 are prohibitive.


Good luck,
       Kirby
> 
> Thanks in advance for answering my questions.
> 
> Best Regards,
> Andrew
> 
> 

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