Pegasus R6 – 1MB Stripe size default?

[Jon Schilling]

I’d contact Promise. They made the product, they would know.

~Jonathan Schilling

[Brad Bussé]

Hrmm, so I left my media drive cloning over to the R6 overnight only to find that only half the data finished and then it had an error. I’ve been trying to render out a 2.5 hour key light Ae sequence, and it gets about an hour or so in then the R6 looses communication with the OS. The drive icon is still mounted and it shows folders, but I can’t open the utility or move any data.

I did some research and found that the R6 by default uses power savings features that can only be disabled via Terminal, so I just did that. I also found that lots of people have had issues with Pegasus drives loosing connection due to RF interference to the Thunderbolt cable connectors, so I wrapped those in tinfoil … we’ll see what happens.

I’m surprised that a professionally-oriented raid array would even park the heads, lower rotational speeds, and spin down the drives. I guess they do that to counter the cheap non-enterprise drives.

I’m considering forcing readahead, is that advised or will that along with disabling the power savings features be stressing out the Toshiba drives too much?

BTW, Jonathan, I finally put in my order for your bare 8-drive unit and I’m sure that that with the Areca card and enterprise drives is going to be a much more reliable unit for the older Mac Pro.

[Chris Murphy]

“stripe size” is a routinely mishandled word, to the point I think it ought to be retired as a term. Some mean it as chunk/strip size, and some mean it as full stripe including parity, and some mean it as full stripe not including parity.

SNIA defines it as stripe depth (same as strip size, same as chunk size which is now deprecated) times member extents less parity extents. The most common synonym for extent that I come across in the Linux storage world is stripe width, which is the number of data “drives”. A six drive RAID 5 array has five data “drives” and one parity “drive.” And for RAID 6 it’s four data “drives” and two parity “drives.” I use “drive” because due to distributed parity, there aren’t dedicated data and parity drives; but functionally you can look at it that way when doing the stripe width (member extent less parity extent) count.

Since 1MB isn’t divisible by either 5 or 6, I don’t know what Promise means by a 1MB stripe size. Maybe they mean 1MB strip size, which would mean it’s a 5MB stripe size. Or maybe they’re rounding up or down a bit, e.g. a 192KB strip size.

There is an optimization here that’s workload specific so the stripe size is important. Normally I’d say bigger is better for video. But there’s another factor, which is something called alignment, and if that’s not ideal then a large stripe size can be bad by causing a lot of unnecessary or inefficient read-modify-write. So basically you just have to test it or ask someone if they’ve tested it or published results with a variety of workloads or benchmarks that do a good job of simulating actual workloads.

[Brad Bussé]

Thanks. I decided to just stick with the 1MB stripe size.

An interesting experience in stability though–my drive kept disconnecting. The icon was still on the desktop, but it just cut connection.

I used the Terminal script to disable power management, but I’m sure that couldn’t have been the issue since it kept disconnecting right in the middle of transferring large sets of data between drives. I read that some people with the Pegasus units have found that they need to wrap the ends of the TB cable provided with tinfoil to block RF interference. I did that and no problems since.

[Chris Murphy]

Disconnects are a different problem, unrelated to stripe size. You need to post the logs somewhere, and/or get the logs to support and have them take a look. It could be as simple a fix as a firmware update, or it might be a hardware defect requiring some hardware to be swapped out. But a disk icon vanishing means it’s not being properly unmounted, and sounds like the physical device is being yanked out from under the OS, and as such your data is at risk of being corrupted. I’d get it dealt with sooner than later.

[Brad Bussé]

No, the icon didn’t unmount. In fact I could still see the directories folders in the root of the drive, just not access any data beyond that. Honestly, since I disabled power management and wrapped the TB cable ends in tinfoil it’s not had a single issue.

[Chris Murphy]

Weird.

Anyway, this sort of thing requires RAID logs, and often kernel messages are helpful also. These days kernel messages get dumped into the system.log and can be filtered by typing the word kernel in the search field. Then what shows up can be copy/pasted into a separate text file and then put up on pastebin. Otherwise it’s like entering a pitch black room and someone says “there’s a problem, what is it?”