Re: RAID 0/RAID 5???

I just have to chime in here and say that RAID5 is not always the best
solution. RAID 1+0, a normal RAID1 array (mirror) once you use more than 2
disks, can offer better performance than RAID5, at the cost of disk space.

Here is an excerpt from a Microsoft white paper entitled "Optimizing Storage
for Exchange 2003"
(http://www.microsoft.com/downloads/details.aspx?FamilyID=c6084d20-9730-4ffc-805d-b957327604c6&displaylang=en)

· Depending on the hardware RAID configuration you use, plan for I/O
penalties. In general, for each write request, hardware RAID generates the
following I/O:

· RAID-0 = 1 write

· RAID-1 or RAID-10 = 2 writes

· RAID-5 = 4 writes



So that means RAID5 has to perform more disk writes for the same data than
RAID0 or RAID 1+0. There's another really interesting table in the MS white
paper "Storage Solutions for Microsoft Exchange 2000 Server"
(http://www.microsoft.com/downloads/details.aspx?FamilyID=5b135312-bbbe-402e-9e2d-3b7b0820d2f4&DisplayLang=en).
It describes the different read and write capabilities for RAID 0, 0+1, and
5. Here's the table:

Table 1 Comparing RAID solutions

RAID solution
Number of drives (cost)
Maximum writes/second
Maximum reads/second
Reliability

RAID-0
10
1000
1000
Low

RAID-0+1
20
1000
2000
Very high

RAID-5
11
275
1100
High



See the difference between writes/sec and reads/sec for RAID 0+1 and Raid 5?
Of course the cost in drives between these two is a factor, and for a server
with only 4 drives, the numbers listed above are no longer the same.

I don't intend for this to be a guide to which RAID setup to pick for
everyone, I just want to make sure your decision is based on the role of the
server, and the disk speed/space requirements. Different RAID configurations
offer different levels of performance and redundancy. For example, a server
with 4 drives running a database program like SQL might benefit more from
two RAID 1 arrays, so that you can separate the database from the
transaction logs (put the two drives on separate drive arrays/mirrors, to
minimize the performance impact on each other).

There are also differences between RAID 1+0 and 0+1, but I'm not going to
get into that! The controllers in the specific systems my company buys don't
have both options, only 1+0.


"Steven Bellamy" <nospam@xxxxxxxxxx> wrote in message
news:eErN4OduFHA.3864@xxxxxxxxxxxxxxxxxxxxxxx
> My 2c worth.
>
> Are you sure that he didn't mean RAID1?
> RAID0 has zero redundancy, so if one disc fails, you lose EVERYTHING!
> RAID1 is a set or identically mirrorer disks, so if one fails, the other
> can carry on working as usual. The read and write speeds usually equal the
> speed of a single disk.
>
> I'd go for RAID5 as it offers great redundancy (although you can only lose
> 1 disk), but you have the added advantage of multiple disk heads reading
> and writing data to and from the array, therefore throughput is usually
> greater.
>
> A useful website that explains RAID configurations can be found here:
> http://www.raid.com/04_01_00.html
>
> Hope this helps.
>
> Kirsten wrote:
>> Hi.
>> I bought two brand new HP/Compaq ML370 servers with 4 SCSI disks each and
>> a
>> Smart Array Controller (hardware for raid capabilities).
>>
>> With my old servers I had RAID 5 with a similar configuration so I was
>> going
>> to configure the new servers with the same mechanism, but the Compaq
>> agent
>> that installed the new computers told me to use RAID 0 because it is safe
>> enough and save lots of CPU+hardware resources.
>>
>> The obvious fact is that RAID 5 is SAFER than RAID 0, but now I'm a
>> little
>> bit confused by this guy's suggestion.
>> So, what do you suggest? Each server will hold an instance of SQL Server
>> and
>> IIS Server.
>>
>> Thanks in advance,
>> Kirsten
>>

.

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