Breaking the ATA Addressing Barrier

BitMan sent in an overview of the next step in addressing large disk drives. I tend to run into these every few years when I try to add a new, large drive to an older machine and find out that some factor is keeping me from being able to use the full drive capacity. Well, the next step will push those limits out quite a ways, giving us a few more years of ever-increasing drive space.

BitMan writes: "If you haven't heard, there has been a new disk geometry limitation looming for some time at 128GB (gigabytes of 2^30 bytes), which is 137GB (gigabytes of 10^9 bytes). As many will note, there have been various BIOS and OS limitations in disk geometry before -- e.g., 512/528MB, 2GB, 8GB, 32/33GB, etc... But what makes the latest 128/137GB "limit" different is that it revolves around the "hard, physical addressing" limitations of the ATA (AT Attachment) interface. 28-bits are used for addressing, which results in the 2^28 sector * 512 bytes/sector = 128/137GB limitation. As such, hardware fiends like myself were wondering when the industry would get around to addressing this "hard" limitation in the ATA interface.

Fortunately, the solution is already in the works. The ANSI ATA committee has accepted a proposal from Maxtor that extends the ATA bus addressing to 48-bits. This allows for up to 128 pB (petabytes of 2^50), which is 144pB (petabytes of 10^15), sizes. This should tide the PC world over until the 2TB (terabytes of 2^40) limit is reached, which is the maximum number of sectors a 32-bit OS can address -- i.e. 2^32 sector * 512 bytes/sector = 2TB.

In addition to breaking the addressing limitation, another addressing limitation was overcome for performance considerations. The maximum number of sectors transferrable in any command was boosted from 8-bit = 256 sectors/command (~128KB max. transfer/command) to 16-bit = 65,536 sectors/command (~32MB max. transfer/command). This should increase ATA/IDE performance in burst transfers and many other operations.

A whitepaper on the new proposal can be found here from Maxtor. Small correction in the article: Maxtor says 144 pB (petabytes) = 144,000 GB (gigabytes), which is quite incorrect. 144pb (petabytes of 10^15) = 144,000 TB (terabytes of 10^12) = 144,000,000 GB (gigabytes of 10^9).

Thanx goes to the most excellent StorageReview site where I first heard of this."

Comments

Enough!

[Waffle Iron]

Why even have "sector"-based addressing when the hard drive is just going to munge the addresses into some other physical layout anyway? It's been sector/cylinder/head compatibility hell for the last twenty years.

Maybe just once they should make the painful switch to a simple flat 128-bit address space and be done with it.

Re:Argh, can't they get it right ONCE

[Anonymous Coward]

There are good reasons for using smaller address words, caching efficiency chief among them. On systems which run their filesystems fully-asynchronously (like linux), filesystem caching efficiency is a primary factor in limiting performance for filesystem-intensive applications. When your file data set exceeds main memory's ability to cache it, performance can plummet like a stone. If your filesystem cache metadata takes up 25% more space because you are using 64-bit address words instead of 32-bit (64 bits is 100% larger than 32 bits, but metadata records contain a lot more than just address words), then your maximum cacheable filesystem is only 80% (1.0/1.25=0.8) what it could have been.

Even with small data set sizes, this can mean a lot, because you see performance degradation when you spill L1 cache, and another when you spill L2. It's a lot easier to get 25% more compact metadata than it is to get 25% larger L1 and L2 caches!

What makes the most sense is to design our operating systems to be able to treat filesystems as large (2+ TB) or small (2- TB), and use cache data structures to match. That way we'll have higher performance in the "common case", and corporations who need to be able to support (eg) huge databases will be able to do so.

For the past few years, hard drive data density per dollar (in best density per dollar products, not top or bottom of the line products) has been increasing exponentially at a rate of 2.15x per year. If we project that naively and assume it holds steady (which is unlikely, so take this with appropriate salt) then we should expect to bump into the 2 TB limit on our home desktop computers in about 10 years. To me, that makes filesystem-segmented 32-bit sector addressing "good enough" for a good long time.

(I have been tracking hardware trends since the early 90's; the past two years' worth has been collected automatically via web-bot from the same vendor, so it is easily indexable .. I hope to enter my hard-copy data some day as well, but what a pain! I have made parts of it, from 1999-01-05 to 2000-09-12, available on my web site here. Collection is ongoing, and I'll refresh the web site from the master records sometime too.)

-- Guges --