What does 6 Gbit SATA mean to you?

Seagate announced hot new disk today: a 6 Gbit SATA interface; 64 MB of cache; and 2 TB of capacity. Time to replace your old disk drives?

No rush Each of these features is a good thing. But only the capacity is usable today.

The faster SATA interface is available on just a couple of high-end PC motherboards. The good news is that the interface is backwards compatible with the 1.5 and 3 Gbit versions. While you don't gain any performance from the faster interface today, you don't lose anything either.

History suggests that the new interface will be fairly common in 12 to 18 months. Since disks have a 3 year useful life you could get 2 years of higher performance if you bought the new drive today -- and buy a new PC in 12-18 months.

Where's the performance? Where and when you actually see improved performance from the higher speed interface is the real question. A sequential read from a 7200 RPM drive can't saturate a 3 Gbit link let alone 6 Gbit.

That is where the 64 MB cache comes in. If the data the operating system is requesting is in the cache, the cache can saturate a 6 Gbit link.

Let's run some numbers. Any 6 Gbit interface is capable of roughly 600 MB/sec of user data after accounting for protocol and encoding overhead (I'm assuming the drivers are well tuned - which may not be true for some time). Delivering 64 MB from cache will take 100 ms, while a sequential read from the desk could take 600 ms.

6X speed up sounds good. But how likely are you to see it?

If you're doing small block or random I/O the answer is "not very." Disk firmware predicts future data requests using the concept of "locality of reference." The idea is that a request for a block of data will be followed by another request near that block. The disk reads ahead and loads the cache with data it expects the operating system to request.

It is a great concept, but with small requests the data transfer time is dwarfed by the I/O system overhead. And if your I/O is really random, locality of reference isn't very helpful either.

What about sequential I/O? Even a 100 MB video file will overwhelm a 64 MB cache. Many disks don't use the cache in sequential I/O because of cache latency. It's faster to skip the cache and go direct from the read head to the SATA interface.

OK, where DOES it help? Seagate tells me there are 2 cases where the larger cache offers noticeably higher performance. The first is in non linear editing (NLE), where multimegabyte video clips are flipping around.

The second is a Media Center PC. There large sequential I/O's are coupled with relatively low audio or video data output rates. The disk can get ahead of the system demand and fill the cache with data that's ready to go.

Luckily for 6gig SATA chip vendors these are high-growth apps.

The Storage Bits take What will really drive demand for the new 6 Gbit interface is the new super speed USB 3 due next year. Capable of 300-500 MB/sec USB 3 will enable a new generation of high bandwidth peripherals.

Seagate's new 6 Gbit interface and larger caches will also become more important as disk drive areal density grows, increasing R/W speeds. Today the disk's higher performance is only important as part of an end-to-end system design capable of processing and delivering higher bandwidth on a sustained basis.

Comments welcome, of course. Update: Commenters have questioned the 3 year useful life, even suggesting I'm a shill for drive vendors. Hardly. I'm a data driven guy. You can review the data from Google and Carnegie-Mellon's Parallel Data Lab in posts I wrote 2.5 years ago: Everything you know about disks is wrong and Google's disk failure experience. Note 1: Annual Failure Rates spike in year 3. Note 2: massive skepticism of drive vendor claims. Note 3: great comments on both posts.

But the term "useful life" means more than "works." It is a combination of reliability, capacity, expense and performance. A Model T Ford might "work" but it is past its useful life, at least in the US. Likewise, a 10 year old 4 GB drive may still work, but the 60kWh you spend each year to run it would buy you a new USB thumb drive that is more reliable with almost no operating expense.

But the biggest issue is that disk drives are mechanical devices and they wear out. Sure, I back up my data 3 ways, but I also replace my disks every 3 years. What is your data worth to you? End update.