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Comment by tomkinstinch | Hacker News Reader

Reminds me of Seagate's Momentus XT, except it sounds like the file management is done in the OS and not on the drive, and that the SSD is not being used as a cache.

http://www.anandtech.com/show/3734/seagates-momentus-xt-revi...

I'm guessing the SSD portion will be MLC?

Are there any other hybrid drives like this in the wild?

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This is not a hybrid drive, it is two drives as evidenced by the picture of the iMac that Phil Schiller showed.

This is more akin to "tiered storage" which has existed before, just not in the consumer space which prevents more/different challenges.

I see, thanks; I haven't had a chance to watch the Keynote yet.

Do you know how difficult it would be to set up something comparable using two drives (HDD+SSD) under, say, Ubuntu?

I don't know about Ubuntu, but apparently Windows Vista and 7 have something very similar. http://en.wikipedia.org/wiki/ReadyBoost

Not even sorta similar, "ReadyBoost" is a cache

"Fusion Drive" provides the union of space.

They are in the same vein of "hide the slowness of rotational media" but that is a pretty huge vein.

In the Fusion Drive example, 3TB of hard drive is combined with 128GB of flash. The union of space is 3.125 TB, compared to the 3TB you would get with a cache.

That's just a 4% increase in space that the Fusion Drive gives you over what a cache would give you. You can call it whatever you want, but I'm going to call them similar.

And "just" a 32x increase in flash size.

Not ReadyBoost, but ReadyDrive:

http://en.wikipedia.org/wiki/ReadyDrive#ReadyDrive

Well, if I wanted to achieve this today and I had an HDD and SSD in this computer, I'd mount /home and maybe /var on the HDD and everything else on the SSD. I usually go with a three partition setup (/boot, /, swap) and even that just with /home on the HDD would get you 3/4ths of the way there.

There are a couple of block-level cache systems in development, but that haven't yet been accepted into the mainline kernel. The one most suited for desktop SSD+HDD use right now is bcache, which is aiming to hit the kernel staging area next year. Facebook's flashcache is the other main contender. Both offer writeback and writethrough caching, but they differ in their policies for what kind of I/O skips the cache. Bcache tries to detect sequential I/O and send that straight to the backing disk, and organizes the I/Os sent to the SSD in a manner that tries to go easy on the SSD's wear leveling and garbage collection algorithms. Flashcache, on the other hand, tries to use the SSD to cache chunks of information that fit the SSD's page size - smaller I/Os skip the cache, which is probably the exact wrong thing for desktop workloads.

I'm surprised that the benchmarks for the Momentus XT are so good, especially since it only has 4 GB (8 for the newer model) of SSD. It would seem that in theory it would be better to just add 4 or 8 GB of DRAM to your main system, but apparently the drive only caches non-sequential data that isn't likely to be read quickly from the platters. I'm thinking of trying one on my aging MacBook Pro.

Anyway the Apple approach should be better because of the bigger SSD and more intelligent caching available at the OS level.

I installed the 750GB/8GB Momentus XT on my late 2008 MBP and it has been a breath of fresh air. Previously I had a 500GB WD Scorpio Black. That was fast but it vibrated a lot and I think battery life was worse than with the Momentus. I mainly use the machine for XCode and Visual Studio and now I am CPU bound.

The first revision of the XT (.5TB) was a dog and barely worth the effort. The new version is significantly improved.

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