RAID your Server's Performance

How do you decide what level of RAID you need to implement? This PCQuest Special Report benchmarks different levels of RAID and compares them

Everyone is familiar with different levels of RAID and that

implementing one of them for your storage would either improve reliability or

performance. However, have you actually analyzed how much of each benefit you

actually get? When you implement an arbitrary RAID level without assessing what

it really provides and try to match that to what you need for your organization,

you get a mess. After sometime, you realize you don't have the reliability or

performance that you actually require and then switching RAID levels on a live

environment is fraught with problems.

Therefore, to help you decide, we conducted a series of

tests on different RAID levels and have come out with conclusions for each

levels based on the results we saw first hand. Our setup used the SkyRunner

server that got the 'Best Web Performer' title from last month's server

review. On this server's SCSI disks, we implemented RAID 0, 1 and 5 one at a

time and stressed the server. All the tests were run once each on Windows 2003

and RHEL AS 4. We used the NetBench benchmark for our tests.

File server performance on RAID level 0 with Red Hat AS		File server performance on RAID level 1 with RedHat AS
File server performance on RAID level 0 with Windows 2003		File server performance on RAID level 1 with Windows 2003

We redesigned both the tests suites to simulate a real

world scenario. We used 19 PCs that simulated a total of 38 clients to the

server. The test was designed in such a way that loading starts from one client

and progressively each of the other 18 join in and try to overload the server in

increments. Finally, from the results, we find out at what point the performance

of the server starts declining.

Level 0



Striping, no redundancy, maximum performance

If you are looking for maximum performance but don't need

any redundancy, you should go for RAID level 0. In this level, you will get

maximum performance both in file services and Web services. In our tests, we saw

that if you used a server as a file server with either OS, you get a throughput

upto 775 Mbit/s. As a Web server, it handles around 3000 requests per second

with RHEL. Surprisingly, Apache running on Win 2003 gave us a poor result of

just around 1200 requests per second. From these results, we conclude that RAID

0 gives you a better performance without redundancy compared to other levels.

For data protection at this level, you will need to deploy a backup solution. If

a disk fails in a striped set, the information would most likely be corrupt.

Level 1



Mirroring, maximum redundancy, minimal

performance loss

When you require redundancy of data with minimal

performance loss, RAID 1 is your choice. This level implements mirroring, where

data on one disk is cloned onto another disk. Here, if one disk fails the other

takes over transparently and data can still be served uninterrupted. In our file

server test, we saw a throughput of 335 Mbit per second on Windows and in RHEL

we saw 487 Mbit/s. Coming to the Web server test, the score was nearly same as

for RAID 0 in both the OS. Overall, our conclusion is that the RAID 1 has

performance issues only with the disk I/O operations, but calculating other

processing-intensive jobs, we don't see any major performance differences. The

only disadvantage in this RAID level is that you have to spare atleast one

additional disk drive for the redundant copy.

File server performance on RAID Level 5 with Windows 2003

File server performance on RAID Level 5 with Red Hat AS 4

Level 5



Striping, balance of redundancy and

performance

If you are looking for a balance of performance and

redundancy both, then you should opt for RAID level 5 and for this, you need at

least three SCSI disk drives. It uses striping to provide redundancy without

sacrificing too much performance. If any one of the drives fail then RAID 5

proactively handles smooth failover switching, without hampering the user's

tasks on the file system. In our tests, it gave us a decent throughput of 326

Mbit/s in I/O operations, on Win 2003.

Surprisingly, at the same level with RHEL the throughput

dropped to 240 Mbit/s. In both the tests, we pulled out one of the disk drives

while the test was in progress to catch any difference in performance. We

didn't find any such drop. In the Web server test, the scores were more or

less as in RAID level 1. This means there is no major performance difference

while doing calculation-intensive jobs. A significant amount of disk space gets

wasted because of the redundant copies. 

Where's SATA RAID?



When you use SCSI disks, you also need a SCSI RAID controller. This can be a

costly affair. Therefore from the affordability angle, people are looking

towards SATA RAID. This is currently the cheapest RAID option out there, since

all motherboards today come with atleast four SATA connectors and inherently

support at least RAID 0 and 1.

The plus point of SATA drives over SCSI ones is that they

come with lots of storage space. Today, you can buy a 500 GB SATA drive, while

SCSI drives still haven't reached that capacity. As SATA drives have become

more affordable and their capacities have also increased, they're incerasingly

occupying entry-level to mid-range servers. They're even being used inside

storage arrays. they've made it cheaper to build multi-Terabyte storage boxes.

When it comes to performance, SCSI is still king. They're

still used in mid-range to high-end servers because of their performance.