Professional Geekism

The woes of incompatible RAID hardware

So in the last few weeks, we’ve been wondering why all these sets of new 500GB hard disks have been degrading in their RAID-5 array, merely hours after being created. Which was also around the time that I’d finished setting up Gentoo, frustratingly.

Eventually it was decided that instead of making frequent trips out to the co-location facility, we’d bring the server in to HQ for some diagnosis. As after three completely different sets of brand-new 500GB disks, purchased from two separate manufacturers, had all exhibited the same behaviour - it was almost undoubtedly a sign that something else was causing the RAID arrays to degrade.

The RAID card used in this particular server is an 8-port 3Ware 9500S. It’s been reliable in the past, and never exhibited a single issue up until the point we began replacing disks with larger, newer models. I even took it upon myself to strip all SATA cables from the machine and replace them with un-used items. Of course, this was a long shot and it made no difference either way (but at least I’d ruled it out).

Now I don’t know who forgot to check - but this particular model of RAID card does not specifically support SATA-II disks. Of course the first ideal that springs to mind is backwards-compatibility; if a SATA-II disk is plugged into a SATA-I port, one would expect it to automatically run at the slower rate (much like PATA of old.) Though as it turns out, that’s just not something you can assume.

So after much head-scratching, Googling and more Googling, I thought it would be worth adding jumpers to the rear of the drives in order to forcibly limit the drive to 1.5Gbit/sec. I’d like to point out that nothing I found on-line, written by either of the two disk manufacturers (Seagate or WD in this instance) mentioning that the jumper limits any other feature of SATA-II - it’s just a speed lock. Kudos to whoever it was that wrote the 3Ware 9500S Wikipedia article (which has been since been deleted), as it proved to be a rather good muse.

For the fourth time I recreated the arrays and began installing Gentoo. After two days of installing and configuring Gentoo, followed by roughly 18 hours of I/O stressing by bonnie++, the RAID card hasn’t skipped a beat. I may be tempting fate by writing about this so soon, but I’m fairly happy to say that I’ve cracked the problem of why our server’s root file system was degrading before it was even fully initialised.

So, how do you use SATA-II disks with a SATA-I 3Ware RAID controller? Forcibly limit the speed to 1.5Gbit/sec, and it all works like it should. As I didn’t find a specific answer to this question on-line, I’m hoping that this may be of some use to others out there who may be struggling with upgrading their 9500S arrays. By all means, please let me know if it has!

Update, 22-07-07: The array is still working a charm, so in the words of Borat - “Great Success!”. Watch it fail now!