What does 6 Gbit SATA mean to you?

What does 6 Gbit SATA mean to you?

Summary: Seagate announced a new disk with a 6 Gbit/s SATA interface, a 64 MB cache and 2 TB of capacity. What do the faster interface and higher cache 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.

Topics: Storage, CXO, Hardware, IT Employment

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  • Transfer from media to controller...

    is the bottleneck. Once the data requested is not in the cache then you aren't looking at any faster transfers than what you have now. Hard drives will always have this issue. Then there are access times. A fragmented 3 1/2 platter can bring your transfers to a crawl. And all of this before it gets to the controller where the 6Gb rate can boost performance.

    More, smaller, faster platters are the only things that are going to give the user sustained performance improvements. Drive makers know this but they also know you are looking at higher costs.

    WD tested larger caches in their Velociraptors and determined the leap from 16 to 32MB provided very little performance gains. I suspect the 64MB cache in Seagate's new drive will provide very little improvement as well. It sounds good and memory is cheap.

    We need to see changes that will make very noticeable improvements with SSD's coming on strong. Smaller(2.5"), faster(10 to 15k+ rpm's) platters is what I would like to see.

  • RE: What does 6 Gbit SATA mean to you?

    The transfer speed will remain meaningless until SATA 10k and 15k RPM speeds become the norm. The speed of the data retrieval from the platters is the biggest factor in drive performance. The disk drive is and will remain the slowest component in your system for quite some time.
  • Bigger deal for SSDs than hard drives

    Sequential read speeds on solid state drives have been banging the ceiling of 3 Gb/s SATA for a while now. 6 Gb/s SATA will give consumer-priced SSDs enough overhead for the near term. I'm still hoping that PCI Express-based SSDs come down from their stratospheric prices, though. No need for silicon-based storage to funnel through a bus made for spinning platters.
    • Yup

      Agree completely. I found it odd that the original article didn't address this. I now have SSDs in my wife's laptop, and am about to switch to one for my work laptop, and will install one as my boot drive on my desktop at home as soon as my Windows 7 disks arrive. This is not quite mainstream yet, but seriously?

      SSDs offer potentially limitless potential for parallelism. Already the 3.5" drives from OCZ with the two 2.5" drives internal hiding behind a RAID 0 controller are flat-lining at the 3Gbps SATA rate. Future SSDs with more parallel channels, or faster performance, or what have you (OCZ is promising a 3.5" drive with FOUR parallel RAID 0 drives internal) will clearly be bottlenecked by the SATA interface. And yes, as I sit there watching my video transcodes take a half-hour, I certainly do care about how long this takes.

      Or just try syncing an iPhone with that "fill empty space with music" feature turned on. After your sync times go over 5 minutes you'll start to care too...
      • Do you mean more than 1 drive on a SATA line?

        Are you talking about RAID 0 Paralleled at the disks and then over a single SATA300 line?

        I can see that being an issue but if that is the case whay aren't you using a SATA line per disk rather than a single SATA line per array?

        I'm currently using 3WARE HW RAID controllers with a single disk per line. Now I could hang a J.Micro JM322 across two drives and out put it to the RAID controller but that really would be taking advantage of the RAID controllers CPU, rather it would just be an aggregate of the simple hardware JM322.

        I that the situation you are talking about?
    • Just Curious....

      What particular SSD are you speaking about?
      I haven't seen any that have achieved any higher than 240Mb/s which still leave some bandwidth on a 300mb/s SATA300 line.

      I'm not doubting your comment I am just curious what drives achieve the kind of bandwidth that will saturate a SATA300 line.
      • Protocol overhead

        I'm going by the benchmarks from AnandTech's epic SSD deep-dive from August. Out of nine tested drives, six are in the 255-260 MB/s range in the 2 MB Sequential Read test. Those drives were a mix of MLC and SLC flash, and Intel and Indilinx controllers. Of the other three, two use the Indilinx controller with cheaper memory (but still move over 240 MB/s), and one uses a crusty old Samsung MLC controller.
    • SSDs are the old ZIPs

      Personally, I think SSDs are that great. Sure they are fast now but their system life isn't that great because they use flash memory and in most cases, flash memory can't survive for 1000+ writes. SSDs are still too expensive and lack capacities.

      Sure the new announced mSATA interface could help a bit but only so much.
      Gis Bun
  • RE: What does 6 Gbit SATA mean to you?

    Where does the three year lifetime com in? My PC at home is mucholder thant three years, and its disk is still operating, of course for days at a time it is not even turned on. In that respect, I suppose a three year 24/7/365 could be a valid time period.
  • RE: What does 6 Gbit SATA mean to you?

    The OS disk cache can easily be 64M or even several times that per disk. Main memory is accessed faster and with much lower to-CPU latency than on-disk cache. The OS/driver knows better than the disk what to cache and how best to interleave/group competing requests.

    What advantage does on-disk cache have over this?
  • RE: What does 6 Gbit SATA mean to you?

    "Since disks have a 3 year useful life you could get 2 years of higher performance if you bought the new drive today"

    Really, and which factual paper did this information come from? Or did it come from the people making these new drives, attached to some "exclusives" if you pushed the fictional figure for them? Or are you trying to pass off the usual 3 year warranty as the lifespan of the drive?

    I have drives that are closer to 10 years old, they still work flawlessly. Where does this 3 year figure come from? 3 years of what level of usage? In 20 years of using PC's I have never heard this figure and I have never replaced a drive because it became useless 3 years later. Sounds like "scaremongering of consumers" if you ask me, which sits in the same box as "irresponsible marketting".
  • RE: What does 6 Gbit SATA mean to you?

    Waiting for RAID enclosures to use it.
  • RE: What does 6 Gbit SATA mean to you?

    Remember that 600 "MB" of disc is 600 * 10^6 bytes and 64 "MB" of cache is 64 * 2^20 bytes.

    67,108,864 bytes / 600,000,000 bytes/sec = .11185 sec = 111.85 ms

    Not that we are counting.
    • Good Catch...

  • What does 6 Gbit SATA mean to ME?

    It means "wait till 16 Gbit SATA to come out." By that time, the price of 6 GB will be more affordable. :-)
  • RE: What does 6 Gbit SATA mean to you?

    Solid State Disks have come down in price and many out-perform traditional drive by a mile. They use much less power and have a lot longer useble life. How would you compare 6Gb traditional drives with SSDs. I think the SSDs win hands down.
    • Except for capacity and $/GB

      Otherwise it is a clean sweep.

      R Harris
  • Absolutely nothing...

    We haven't even saturated SATA 1 yet.

    This is all just marketing chest-thumping.