*two* RAID arrays better than one?

From: Tom S. Rodman <rodmant_at_solaria.sol.net>
Date: 1998/05/05
Message-ID: <6in408$cql@ns.sol.net>#1/1

We're about to install a new/better hardware RAID card in our quad pentium pro HP LXePro Windows NT Server (a 3 channel NetRAID card). Should we:

use our four 9GB drives in a single RAID 5 array
buy 2 additional 9GB disks and set up 2 separate RAID arrays, each with three 9GB disks

The server has an informix database engine, and also acts as a file server to about 40-80 users. We're thinking of placing the NT operating system and the user filesystem/fileserver data on one RAID array and the informix database on the other RAID array.

We know that there are reoccuring longish operations which read from the informix database and write to the filesystem. There are some operations that work in the other direction (long reads of user data on the filesystem, which then generate writes to the informix database).

Here's an argument for for 2 separate RAID arrays presented by a member of our team:

Here's some quick math on 1 vs 2 arrays (given a few assumptions) ...

Assume an average Disk I/O rates of 6MB/sec; ex: the outside of the plater is typically faster than the middle and rates can vary from 7.5 to 4 MB/sec.

Assume a 4.5ms seek time; half the 9ms time listed for some disks.

A 16MB file copy might be copied using 16KB blocks, thus requiring 1000 reads and 1000 writes.

ONE ARRAY

16MB @ 6MB/sec => 2.67 seconds
2.67 second * 2 (Read & Write) => 5.33 seconds reading & writing

1000 head seeks * 4.5ms/seek => 4.5 seconds 4.5 seconds * 2 (Read & Write) => 9 seconds seeking for R/W

 TOTAL: 5.33 + 9 = 14.33 seconds for I/O
		^^^^^

TWO ARRAYS

2 seeks (1 per array) => 9ms
2.67 seconds reading/writing => 2.67 (ie. I/O in parallel) 0.1 seconds (WAG) for overhead of reads & writes in parallel

 TOTAL: 2.779 seconds
     ^^^^^

CONCLUSION

Some I/O can be drastically reduced when two arrays are used; (ex: 14 secs down to 2.8 seconds).

DISCLAIMER: The type of I/O we typically see and how RAID arrays, cache, etc. effect it all will vary results; we can in no way assume that we will these results.

Can anyone poke any holes in the above argument? Are two RAID arrays going to give us better performance than one?

Please also respond by e-mail.

                                     
		       thanks,

                       Tom Rodman 
                       Johnson Controls, Inc
 
                       1-414-274-5041/fax:4400
                       tom.rodman_at_jci.com

Received on Tue May 05 1998 - 00:00:00 CDT