# # Understanding RAID
APLawrence.com -  Resources for Unix and Linux Systems, Bloggers and the self-employed

I've removed advertising from most of this site and will eventually clean up the few pages where it remains.

While not terribly expensive to maintain, this does cost me something. If I don't get enough donations to cover that expense, I will be shutting the site down in early 2020.

If you found something useful today, please consider a small donation.



Some material is very old and may be incorrect today

© March 1999 Tony Lawrence

Raid

March 1999

Just a very few years ago, RAID was an expensive option. That's all changed, and today anyone with high disk performance needs or concerns about data reliability should consider some sort of RAID configuration.

Hardware RAID is transparent to the operating system. There will be some utility either in the BIOS or installed as an application that lets you see through that transparency, but the operating system itself only sees the logical drive that the RAID presents.

This might mean, for example, that three physical disks look like one to your operating system. Anything that references disks will (/etc/conf/cf.d/mscsi on SCO Unix, /etc/fstab on any Unix/Linux) will only appear to have ONE disk drive.<

Often RAID controllers present each array as the id of its first member, or they'll just go in sequence, first array seen as id 0, second as 1, but yours could be different. So, if you looked in /dev, you'd see one entry as though there was one physical drive. If your OS creates entries that represent SCSI id's and your first physical disk was ID 0, that might be what you'd apparently see in /dev: a drive that looks like a physical ID 0 device as far as the OS knows, but of course it really isn't..

Software raid is slightly different, but at the end it all comes to the same place: your OS sees what the RAID software wants it to see. With software raid, you can get at the physical disks without any special utility, and the device node used for the RAID will be different than any physical pointer. See Linux Logical Volume Manager (LVM) on Software RAID for an example.

RAID means "Redundant Array of Inexpensive Disks". There are basically 5 defined levels of RAID:


As alluded to above, RAID can be implemented in hardware or software. There can also be configurations that are really both: Sun's high end RAID products are tightly coupled software and hardware.

It used to be that RAID was always SCSI based. That's no longer true; inexpensive IDE RAID configurations are now available. They are not going to have the performance characteristics of a SCSI design, but they cost less, and certainly would give good value for the money.

March 1999 Anthony Lawrence. All rights reserved.




Just a very few years ago, RAID was an expensive option. That's all changed, and today anyone with high disk performance needs or concerns about data reliability should consider some sort of RAID configuration.

RAID means "Redundant Array of Inexpensive Disks". There are basically 5 defined levels of RAID:


As alluded to above, RAID can be implemented in hardware or software. There can also be configurations that are really both: Sun's high end RAID products are tightly coupled software and hardware.

It used to be that RAID was always SCSI based. That's no longer true; inexpensive IDE RAID configurations are now available. They are not going to have the performance characteristics of a SCSI design, but they cost less, and certainly would give good value for the money.

March 1999 Anthony Lawrence. All rights reserved.


If you found something useful today, please consider a small donation.



Got something to add? Send me email.





(OLDER)    <- More Stuff -> (NEWER)    (NEWEST)   

Printer Friendly Version

->
-> Understanding RAID

1 comment


Inexpensive and informative Apple related e-books:

Digital Sharing Crash Course

Take Control of Preview

Take Control of Apple Mail, Third Edition

Are Your Bits Flipped?

Sierra: A Take Control Crash Course





More Articles by © Tony Lawrence







Fri Sep 18 17:03:27 2009: 6933   TonyLawrence

gravatar


(link) "RAID's Days May Be Numbered"

Quote:
The hard error rate for disk drives has not, for the most part, improved with the density. For example, the hard error rate for 9GB drives was 10E14 bits, and that error rate has increased an order of magnitude to 10E15 for the current generation of enterprise SATA and 10E16 for the current generation of FC/SAS drives. The problem is that the drive densities have increased at a faster rate.


What this means for you is that even for enterprise FC/SAS drives, the density is increasing faster than the hard error rate. This is especially true for enterprise SATA, where the density increased by a factor of about 375 over the last 15 years while the hard error rate improved only 10 times. This affects the RAID group, making it less reliable given the higher probability of hitting the hard error rate during a rebuild.






------------------------


Printer Friendly Version

Have you tried Searching this site?

This is a Unix/Linux resource website. It contains technical articles about Unix, Linux and general computing related subjects, opinion, news, help files, how-to's, tutorials and more.

Contact us


Printer Friendly Version





Whenever the literary German dives into a sentence, that is the last you are going to see of him till he emerges on the other side of his Atlantic with his verb in his mouth. (Mark Twain)




Linux posts

Troubleshooting posts


This post tagged:

Basics

Disks/Filesystems

Install/Upgrade

RAID



Unix/Linux Consultants

Skills Tests

Unix/Linux Book Reviews

My Unix/Linux Troubleshooting Book

This site runs on Linode