@Proton Wrangler

> Self. Foot. Shoot.

> Enjoy the extra 20% capacity while you can.

See: http://queue.acm.org/detail.cfm?id=1317400 where it says...

DB What are the provocative problems in storage that are still outstanding, and does ZFS help? What’s next? What’s still left? What are the things that you see down the pike that might be the big issues that we’ll be dealing with?

JB There are not just issues, but opportunities, too. I’ll give you an example. We were looking at the spec sheets for one of the newest Seagate drives recently, and they had an awful lot of error-correction support in there to deal with the fact that the media is not perfect.

BM They’re pushing the limits of the physics on these devices so hard that there’s a statistical error rate.

JB Right, so we looked at the data rates coming out of the drive. The delivered bandwidth from the outer tracks was about 80 megabytes per second, but the raw data rate—the rate that is actually coming off the platter—was closer to 100. This tells you that some 20 percent of the bits on that disk are actually error corrections.

BM Error correcting, tracking, bad sector remapping.

JB Exactly, so one of the questions you ask yourself is, “Well, if I’m going to start moving my data-integrity stuff up into the file system anyway—because I can actually get end-to-end data integrity that way, which is always stronger—then why not get some additional performance out of the disk drive? Why not give me an option with this disk drive?” I’ll remap all the bad sectors, because we don’t even have to remap them. It suffices to allocate it elsewhere and basically deliberately leak the block that is defective. It wouldn’t take a whole lot of file-system code to do that.

Then you can say, “Put the drive in this mode,” and you’ve got a drive with 20 percent more capacity and 20 percent higher bandwidth because you’re running ZFS on top of it. That would be pretty cool.

DB That’s a really exciting idea. Have you had those discussions with the drive vendors about whether they would offer that mode?

BM Not quite, because they’re most interested in moving up the margin chain, if you will, and providing more unreliable devices that they sell at a lower cost; it isn’t really something they care to entertain all that thoroughly.