Managing RAID on Linux

Posted by timothy on Wednesday February 12, 2003 @04:00AM from the or-hardly-workin' dept.

rjnagle writes "The availability of HOW-TOs and newsgroups is supposed to make the sysadmin's job easier, right? Much as I am a proponent of the 'distributed learning model' for Linux, the endless searching for answers on the Web for setting up Linux RAID was getting to be a royal pain. Sure, there was a RAID how-to and an excellent newgroup, but some of the information is out of date, and the tricks suggested by people a year ago may be no longer needed today. Robert reviews the O'Reilly title Managing RAID on Linux below to see how it stacks up to HOWTOs, guesswork and anecdotal evidence. Managing RAID on Linux author Derek Vadala pages 245 publisher O'Reilly rating The best reviewer Robert Nagle (aka idiotprogrammer) ISBN 1565927303 summary This book brings RAID to the masses

A person deciding to go with RAID faces a panoply of options and gotchas. Hardware or software? How many controllers? ATA or SCSI (or ataraid)? RAID 1 or RAID 5? Which file system or distribution? Kernel options? Mdadm or raidtools? /swap or /boot on raid? Hybrid? Left or right symmetric? One poster pointed out that putting two ATA drives on the same controller could impact performance. Yikes! Didn't I do that? Upon discovering that O'Reilly had just published its Managing RAID on Linux book, looking at sample chapter , I bought the book and let my blood pressure return to normal.

RAID is one of these subjects that is really not complex; it's just very hard to find all the information in one place. This is precisely the book to solve the problem. Author Derek Vadala, sysadmin and founder of Azurance.com, an open source/security consulting firm, has gathered a lot of information and even personal anecdotes to go through the decision making process when going over to RAID. He goes step-by-step through that process, educating us about hard drives, controllers, and bottlenecks along the way. This exhaustive book may be the first to bring RAID to the masses.

Although parts of the book (RAID types, file system types) may seem already familiar to experienced Linux users, it is helpful nonetheless to have everything in a nifty little book. A section of file systems provided not only a rundown of the merits and drawbacks of each one, but also a guide to their utilities. I learned for example what "file tails" for Reiser are, and why using them causes performance to degrade after reaching 85% capacity. The book compares raidtools with mdadm as well as lovely commands like nohup mdadm -monitor -mail=paranoidsysadmin@home.com (which, if you haven't guessed, causes the system to email you RAID status reports upon boot).

People who use software RAID may skip over the chapter on RAID utilities for the leading RAID controller cards. Still, there was one interesting tidbit: Why, the author asks, do makers of controller cards put all their BIOS utilities on DOS floppies which require us to find a DOS boot disk? Seriously, how many of us carry around DOS boot disks nowadays? The book made me aware for the first time of freedos, an open source solution that solves precisely that problem.

The Software RAID stuff was pretty thorough and clarified a lot of things. The book does an excellent job in helping to identify and eliminate bottlenecks and optimizing hard drive performance (using hdparm and various monitoring commands). The anecdotes and case studies definitely clarified which RAID solution is suited for which task.

I am less impressed by the book's sections on disaster recovery and troubleshooting. Although these subjects are brought up at several places in the software RAID chapter, the book could have discussed several failure scenarios or used a fault tree (such as the famous Fault Tree in Chapter 9 of the Samba book, a marvel for any tech writer to read). The book doesn't even discuss booting with software RAID until the last 10 page of the book and then gives it only a single paragraph (even though the author acknowledges it as "one of the most frequently asked questions on the linux-raid mailing list."). Call me old-fashioned, but isn't the ability to boot into your RAID system ... kinda important? As someone who just spent a significant amount of time troubleshooting RAID booting problems in Gentoo, I for one would have liked more insight into the grub/lilo thing. Also, in the next paragraph in the last chapter on page 228, the author casually mentions that "all /boot and / partitions must be on a RAID-1." Say what? Please pity the poor newbie who religiously follows the instructions in the book but fails to read until the end. I'm not sure what the author meant by this statement, but it required a much more substantial explanation and needed to go into a much earlier chapter.

These complaints don't detract very much from this excellent book, a true O'Reilly classic and a model of clarity and helpfulness. This book provides enough knowledge to avoid the dread and uncertainty that comes with trying to tackle Linux RAID. With a book like this, a sysadmin can sleep a little easier.

Recommended Readings:

Reliable Linux , by Iaian Campbell, Wiley and Sons, Dec 2001, ISBN: 0471070408. Gives excellent information not only about RAID but on general Linux reliability issues.
Software RAID in the Linux 2.4 Kernel by Daniel Robbins. (Part Two).
Linux Journal Article on Software RAID by Joe Edwards, Audin Malmin and Ron Shaker. ( Part Two).
"How to do a gentoo install on software RAID" by Chris Atwood. Gentoo User Forum.

Robert Nagle (aka Idiotprogrammer )is a Texas technical writer, trainer and Linux aficionado. You can purchase Managing RAID on Linux from bn.com. Slashdot welcomes readers' book reviews -- to see your own review here, read the book review guidelines, then visit the submission page.

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