What do I need to saturate a 6Gb/s SATA connection?

3

I know the throughput of a standard HDD doesn't quite saturate a 6Gb/s connection (if someone has a numerical value, that would be great). I have been told that the only way to reasonably saturate this is to put some SSDs in a RAID. How true is this?

EDIT When designing a configuration, what are good reference numbers (MB/s) to associate various HDDs and SSDs with (7200 rpm, 10000rpm; MLC, TLC, SLC)? When looking on a site such as NewEgg, there is no real speed listed, just the compatible SATA Ports (SATA III 6Gb/s)

Jeff

Posted 2013-01-11T17:14:29.007

Reputation: 133

For your edit, if you want to know the write and read speed of hard drives, I think you need to look at benchmark tests. SSDs have such info directly in the specs, but HDDs don't. Probably, they don't have to and it's a pain to test (or painful for sales to reveal :p), so they don't test or show the results. Search for "<HDD1> vs <HDD2>" or "hard drive benchmark" or "<HDD> benchmark". I guess it's a bit like processors. They say what they want, but the end result isn't always exactly what you expect from the specs. RPM are vaguely an indication of physically possible speed, but zero reliable. – Ariane – 2013-01-11T18:08:57.830

Good point. I had intended on vaguely designing my new setup with estimated numbers, then looking up benchmarks to fine tune my design. It wouldn't be a bad idea to start with researching benchmarks. – Jeff – 2013-01-11T18:43:21.477

Mhm. I haven't looked for HDD benchmarks, but for CPU ones, you have graphs of which are best, with the price as well, and price/quality rate graphs too. So you can basically do your shopping there. Unless you find a super interesting special deal. :p – Ariane – 2013-01-11T18:53:20.610

@Ariane RPM should be multiplied with density per platter, as well factor in the number of platters. – Hennes – 2013-01-12T00:06:03.120

@Hennes: R-right. *Feeling very unknowledgeable* – Ariane – 2013-01-12T01:43:03.143

Just think of it like this: Say I read a book at one line per second (one line starting at the right of a page, ending at the left side of a page). If that line has 80 characters on it then I read 80 characters per second. It is has a smaller font with 120 characters then I read 120 characters per second – Now to a disk. – A drive rotating at 7200 RPM rotes 7200 times per minute. Or 7200 times per 60 second. One rotation takes 60/7200 seconds (0.0083sec, or 8.3 ms). So I read one 'circle'/line per 8.3 ms. Higher density (smaller 'fonts') allow faster reading. – Hennes – 2013-01-12T01:59:39.000

Answers

3

How true is this?

Partially. A SATA-III 6.0gbit per second connection has a maximum throughput around 550MB/sec.

Anything on a SATA-III bus which can deliver those settings on a single channel will max that out. Right now there are only two common setups which reach those speeds:

  1. A fast SSD (RAID does not factor in this, since it is per channel, and you would put each SSD on its own SATA channel).
  2. Port multipliers (which fan out to multiple SATA devices. Either SSD, HDD, or a combination).

Edit, answering the OP edit:

Current convential HDD speeds are:

  • 2½ inch laptop drives: about 60-90mb/SEC
  • Desktop drives: around 100MB/sec
  • 15k RPM high end enterprise SAS drives: Up to 200MB/sec on the outer tracks

Curent SSD speeds can max out a SATA-III channel on higher queue dept sequential reads.

For current day SSD speeds, check benchmark sitrs or generic hardware sites. (e.g. Toms Hardware, though that one is slowly moving from an informative site to a clearly sponsored site).

Hennes

Posted 2013-01-11T17:14:29.007

Reputation: 60 739

Additionally, I find it funny how we have fast cables (and I'm not even sure SATA III is the fastest we have), so fast that most devices can't use its full power. Cool in a way, to know things can evolve without too many bottleheads downstream. – Ariane – 2013-01-11T17:21:00.350

Fast designs then current generation devices need just means it will last a few years before we have to change/upgrade. – Hennes – 2013-01-11T17:22:26.143

In response to 1. Would you not be able to put two SSDs in a RAID 0 configuration to obtain faster speeds on the same channel? – Jeff – 2013-01-11T17:34:32.047

@Jeff I believe the RAID controller puts the bar higher, as well. So yeah, more data is going through, but more data is able to go through as well, so still no saturation. I've never seen RAID with my own eyes, but I figure it's not plainly two drives plugged into one motherboard SATA output through a controller, yes? That would be a dumb splitter. – Ariane – 2013-01-11T17:39:17.890

@Ariane and even our "fast" cables are truly slow by RAM or cache standards. Also, you can purchase PCIe-based solid-state or DRAM drives to achieve much higher speeds than one could get over SATA. – Breakthrough – 2013-01-11T17:46:01.290

@Breakthrough Heh, yeah. Whenever you think something is awesome / the best, when it comes to computers, there's always better. – Ariane – 2013-01-11T18:04:01.053

@jeff the only way to put more SATA devices on the same cable/channel is via a port multiplier. If you use two cables then without one you end up on two different serial channels. – Hennes – 2013-01-11T18:32:03.460

@Hennes It seems my understanding of RAID card vs PM is lacking. After a quick search, it looks like the most common implementation of hardware RAID (in a desktop setup) utilizes the PCI slot to communicate to the computer vs PM which utilizes the SATA ports. It seems that is the main difference as I also read a note that mentioned RAID software is available for some PMs. Is this correct? – Jeff – 2013-01-11T18:51:27.320

1A hardware RAID card communicates via the motherboard though a PCI or PCIe slot. Thus it does not use te motherboards SATA ports at all. Depending on the card it will either supply its own SATA or SAS ports. Each of those is often connected to a drive or to a port multiplier. – Hennes – 2013-01-11T18:57:45.017

@Hennes Good information. Do you by chance have a source for the speed information above? – Jeff – 2013-01-11T19:11:53.560

It should also be noted that the average consumer SSD will NOT saturate that bus, even after you factor in the protocol overhead. – user11934 – 2013-01-11T19:39:00.663

@jeff: For consumer drives: Own experience in 3.5 inch 7200 RPM SATA drives (in this case mainly 1TB drives). For laptop drives: benchmark sites and own experience (though my own d2.5 inch drives are older and are closer to 50MB/sec). For the 200MB/sec: Seagate website promoting their newest latest fastest (15K RPM, SAS) drive. For SSD's. Mostly TomsHardware and Anandtech. (E.g. http://www.tomshardware.com/reviews/ssd-recommendation-benchmark,3269.html )

– Hennes – 2013-01-12T00:02:04.907

@ user11934 Average users on desktops do not generate a high queue depths. And most reads will be semi-random 4k-ish in size. But a relative large modern SSD will often top out a SATA-II (3.0GB/sec, approx 270MB/sec) bus, which means SATA-III is useful. – Hennes – 2013-01-12T00:04:19.467

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