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Gerry Patterson, The World's Most Humble Blogger

Disastrous DMRAID


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Date: Sun, 01 Mar 2015 21:30:08 +1100

The Ubuntu dmraid package offers support for ATA RAID (Sometimes referred to as "Intel or SATA FakeRAID"). Recent Server distros detect FakeRAID BIOS and offer to automatically install the dmraid package to support it.

RAID is an acronym for Redundant Array of Inexpensive Disks. In theory, it would seem that it would always be a good idea to install a RAID for a server rather than single disk drives. However after many years of disasters I have decided to eschew Ubuntu dmraid.

My past experience with true RAID has been good. For PCs and mid-range machines a true RAID will have a dedicated controller with on-board firmware. In these cases I have seen all the benefits that proponents of RAID say we can expect.

However FakeRAID is often just two SATA disks and the RAID functionality is supplied by the OS driver. I have experienced far less satisfactory performance with FakeRAID ... And in Linux, over the past 15 years, I experienced some truly monumental disasters.

The reason it took me so long to learn these bitter lessons in the school of hard knocks was the large body of online opinion and advice regarding the superiority of RAID. I thought there must be something odd about my experience ... Perhaps it was out of the ordinary ... Maybe it was just plain old "bad luck" ... However the last disaster was so terrible that I decided that evidence was far more persuasive than terabytes of opinion to the contrary.

The lesson I have learned is that dmraid in Ubuntu is flakey and unreliable. Any power outage (even a hard reboot initiated by pressing the reset button on the front of the box) is likely to be fatal for one or both of the disks in RAID 1 (mirror). Not only is the RAID less reliable but performance is considerably slower than the native disks

And after all the bitter experience with disk failures, I have only successfully re-built a mirror once. Every other disaster has required restoring and often replacing a disk.

The native disk with ext2/ext3/ext4 filesystems are very robust. Often they can withstand the most hideous power outages, and require nothing more than a reboot. Sometimes there are errors, but usually they can be fixed with fsck (or e2fsck for newer systems). This has been my experience for desktops and servers (Intel and/or AMD PCs used as servers)

The new SATA drives are especially reliable (provide you don't run them with Ubuntu dmraid).

Of course I don't want to argue the point. My opinions have been formed in the crucible of past experience --- 9 dmraid catastrophes, 6 of them caused by power outages vs only one failed native drive. And I must say that one that failed was a rather dodgey old drive, that gave me plenty of prior warning that it was on its way to the disk graveyard.

As I said, In this particular instance I was a slow learner ... But I have been beaten around the head enough by the evidence ... Which is frankly over-whelming. After the most recent disaster, I removed dmraid from any machine for which I had administrative privileges. This freed up an extra drive (which could be used to make hot backups), improved performance and reliability.


Note: I will admit that FakeRAID seems to be more reliable on Windows machines. This may be because Microsoft have a close working relationship with Intel, and have been given access to proprietary information regarding the SATA FakeRAID interface. Information that might not be available to those who have to rely on officially published APIs. Or it may be that the PC FakeRAID has been optimised for Windows operating system.


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Copyright     2015, Gerry Patterson. All Rights Reserved.