WD Caviar Green: Different RPMs, RAID5 suitable?

Currently I have a 3Ware 9500S-12 with two Caviar Green 1TB disks in RAID1. This isn't enough space, so I want to buy another two disks and run a RAID5. This isn't mission-critical (my media server, hence the space requirement), but I move a lot of large files between my machines and the server. I wasn't able to pin down for sure the RPM of the WD drives I'm using; they could be 5400, 7200 or anything in between.
Would it matter much for a hardware RAID5 if the disks are different speeds? I'd prefer it if they were for obvious performance reasons, but as long as it's stable, I guess it would be fine. The disks are 1x WD10EARS-00Y5B1, 1x WD10EADS-11M2B2 and potentially two of WD10EARX, if this helps. Thanks.

raid

Gargravarr

Posted 2012-04-30T18:44:06.547

Reputation: 279

1WD does not support using Green drives in a RAID at all. So you are already being risky. In any case I strongly suggest you do not use RAID5, and instead go for RAID10, or RAID6. That way you will be at least somewhat safe when one of those drives failes. – Zoredache – 2012-04-30T19:16:26.080

1Using Green Drives is a mess. Trust me. They can work fine but they can cause a ton of issues down the road. I have personal experience with this. As far as using RAID10 that is nearly as risky as RAID5 since 2 disk fails could cause data loss. RAID6 would be the best option. But if you get abnormal slowness it's likely due to the Green drives, Get black eventually IMO. – Jeff F. – 2012-04-30T19:42:56.957

I've had the Greens in a software Mirror for over a year, and they're now in a hardware. I understand there are risks with the disks being kicked off hardware RAIDs because of timeouts, etc, but are they really so risky? Also, RAID6 and RAID10 have the same 2-disk failure tolerance; 10 is only fatal if it's the same side, granted. Sadly the controller doesn't support RAID6 and I need more than 2TB of space. – Gargravarr – 2012-04-30T20:05:08.983

The green drives' performance characteristics make them I'll-suited for a RAID. Another risk is that rebuild times are longer with larger disks, thus the chance of a second failure before the rebuild completes is also higher. Also keep in mind that any RAID configuration with parity is also susceptible to the "RAID 5 write hole," which can cause data loss. – rob – 2012-05-01T05:53:06.507

Answers

I depends on the controller I think, but the more advanced controllers will just run the disks the disks at the speed of the slowest disk. It's not a good idea though.

Lucas Kauffman

Posted 2012-04-30T18:44:06.547

Reputation: 2 545

From the perspective of "Can I make a RAID 5 from these disks, even though the RPMs may vary", the answer is "yes." From a perspective of "Will it matter?", the answer is "It depends. Do some benchmarking of IO on your system and see if you have a problem." We don't know if this server is for 300 heavy-use clients at a corporate training center, or for 5 home machines. (And if it's for the latter, you're on the wrong site.)

mfinni

Posted 2012-04-30T18:44:06.547

Reputation: 526

It is the wrong site. He wrote about his "media server" ... – Sven – 2012-04-30T18:49:34.943

Doesn't mean that it's not for an office that consumes media - in my example, a corporate training center. I'm open to the idea that this is for work, although it's likely it isn't. – mfinni – 2012-04-30T18:50:11.920

Sorry, didn't realise there was a different site for home-specific servers. It's a home media/everything-else server. – Gargravarr – 2012-04-30T18:54:24.943

OK - yeah, please read the FAQ. We'll vote to move this to SU (SuperUser), the appropriate site for home users. – mfinni – 2012-04-30T19:05:36.567

Thanks for moving it, sorry for the trouble. IO load on the machine is not high, and it's mostly me and a streaming client using the machine, though I do pass gigabytes between my computer and the server regularly. – Gargravarr – 2012-04-30T20:03:04.477

Your drives are labeled as having 'intellipower,' and WD isn't clear on what that means. I looked up your drives and found sites listing them as 5400, 7200, and 5400-7200, so it's possible that the HDD changes rotation speed depending on your needs.

Disks in a RAID array don't have to be the same speed; they don't even have to be the same size, since the controller should decide that the maximum possible size of the array will be based on the smallest hard drive. What ought to happen is the controller will send data to be written, and it'll wait until all of the hard drives are done before sending more data to be written. Thus it'll essentially wait for the slowest drive.

I think the best course of action would be to contact 3Ware and ask if the controller will work with different speed hard drives, though I'm sure it's not a problem.

I wouldn't expect different spindle speeds to cause a problem because different hard drives with the same spindle speeds can have different data transfer rates, and even identical hard drives will have varying write speeds as they write. Yet there's no problem mixing these drives.

edit: I also did a quick check online and it sounds like other people mixed different spindle speed drives without a problem.

Amin Itani

Posted 2012-04-30T18:44:06.547

Reputation: 21

Thanks for the info, I'll research the controller. I had a problem with a PowerEdge last year, accidentally using SCSI drives of different speeds; the RAID5 collapsed, but was fine when I used matched disks, hence the question. – Gargravarr – 2012-04-30T20:01:48.317

Purely anecdotal remarks, so take them for what they're worth.

A company I worked for used WD Caviar Greens for their attractive price point and lower power consumption for a number of years, and used them in both single-drive setups as well as multi-drive hardware-based RAID (1, 5, 10). Primarily the controllers were 3ware 8xxx and 9xxx series with some newer LSI models as well. We often found that the drives would be kicked out of the array either showing FAIL or more commonly just ERROR. Both cases would lead the array to become degraded, however in the latter case one could simply issue an array rebuild and the drive would pick right back up and run for quite some time without issue.

After this happened a few times on different controllers/servers, we suspected something was up with these Green drives. We dug around in the spec sheets (WD doesn't make it clear in the least from their product briefs) and found that these drives operate at 5400rpm and that is where the power savings come from. What we concluded is that the controller was seeing these units as bad when they took longer to respond than the controller expected and subsequently were dropped from the array by the controller.

Also a likely contributing factor is the lack of TLER support in these drives, so anytime there was a bad sector, the drive would also be dropped.

Furthermore, when compared to Caviar Blacks or the RE4 drives, there is a very significant IO imporvement in our tests in a 4 drive RAID10 array.

Garrett

Posted 2012-04-30T18:44:06.547

Reputation: 4 039