Re: RAID 5 performance

Paul Baumgartel <paulb_at_pcnet.com> wrote:

>I've just done a direct comparison of RAID 5 and non-RAID disk configurations.
 

>All disks are mounted on the same CPU, a Digital Alpha running OpenVMS 6.1. The
>non-RAID configuration contains a database spread across 4 2-GB SCSI disks. The
>RAID 5 configuration consists of 4 data disks and 1 parity disk as a single logical
>volume, with all database files on that volume. In both cases, the online redo logs
>were on a separate disk from all of the other database files. I created identical
>databases on each of the two configurations, with two Oracle instances to access
>them.
 

>The application that I tested was a bulk data load. Part 1 of the load uses
>SQL*Loader to load data from a set of ASCII files into staging tables. Part 2 of
>the load executes a number of packaged procedures to find inconsistent data, compute
>various values, generate primary keys, and create new versions of updated data; it
>then loads the results into the actual application tables. This part of the load
>does a significant amount of reading from the database as well as writing.
 

>I tested the load on each database, with only the database instance under test
>running. On the non-RAID array, the entire process, for one set of files, took an
>average of 12 minutes. On the RAID 5 array, the average time was 32 minutes.
 

>The write performance seemed particularly bad, as evidenced by write response times
>observed in the SQL*DBA file I/O monitor. Response times ranged up to 150 ms on the
>RAID, where typical values on the non-RAID disks were in the single digits.
 

>YMMV, but this makes it pretty clear to me that, for this application at least, RAID
>5 is not the way to go. Next I'm going to test RAID 0+1 (striped and mirrored), and
>I'll post the results.

Having researched into RAID 5 some more, and having found out that the RAID 5 you are using is on hardware rather than within VMS I can offer the following explanation, hope this helps.

RAID 5 is very, very bad if you are performing many writes, the reason for this is that the data is written first, then it has to be read back from the disks in order to create the checksum/parity data, then it has to write this data back to the disks as well. Thus in order to write data it actually performs two writes and one read. However, your read times should be reasonable. RAID 5 was a compromise to offer data integrity, and speed was never really the issue. As for your RAID 0+1 experiment, it should perform pretty well whilst offering the best integrity, I'll be very interested to see any data you get,

Ashley Received on Wed Nov 15 1995 - 00:00:00 CET