SATA-3

ATA, or 'Advanced Technology Architecture' was the favourite way to connect internal hard and floppy drives up inside a desktop PC. SATA, or Serial ATA is trying to keep up with a changing world, but the old SATA connectors are not siutable for laptops and tablets, and flash drives need much faster connection speeds than old SATA-2 could provide. The SATA-3 standard was released in May 2009 and tries to address some of these issues. It has since been enhanced by extras like eSATA and mSATA.
While SATA can be considered equivalent to Serial Attached SCSI (see the SAS section of the SCSI page for details) it seems that all the enterprise storage vendors are now using SAS for their low tier storage.
So is SATA going into a slow graceful decline, or will the enhancements keep it going for a few more years yet?

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Data transfer rate

SATA-2 speed is 3.0 Gb/s = 375MB/s burst speed and 300 MB/sec sustained speed.
SATA-3 speed is 6.0 Gb/s = 750 MB/s burst speed and 600 MB/s sustained speed.
SATA3.2 introduced SATA Express, which uses SATA software protocols over the PCI Express hardware interface to increase SATA transfer speeds up to 8 Gbit/s or 16 Gbit/s.
The SATA2 speed is adequated for most physical disks, SATA-3 is only really needed for solid state disks.

   

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Port multipliers

Port Multipliers are designed to make it easier to add a number of devices to a SATA cable by using a simple hub for port expansion. It allows Port Multipliers to notify the host if a device has been plugged or unplugged from a port, eliminating the process of host polling to determine where devices have been added or removed. Port multipliers provide similar functionality to SAS expanders, though the architecture is different. A given SATA domain can have a maximum of 15 target devices, one for each port. The 16th port is used as a reserved control port that can be used to collect information such as the number of device ports connected.

The PM drivers contain address information within their data frames, to ensure that they can address the correct PM port. Legacy BIOS systems do not contain that information. However it is possible to perform a BIOS boot from a SATA device, but BIOS will only address port 0. Once the boot is complete, a PM driver can be loaded that will address the other ports.

Port Selectors

SATA can only have one path active at a time, which is a single point of failure. A port selector is basically a failover switch that will switch in a fail-over path to storage devices if the primary path fails. A Port selector is a 2-input-to-1-output failover switch that will switch a storage device to a fail-over path to if the primary path fails. Port selectors are small and inexpensive and are manufactured within the SATA hard disk caddy or actually on the disk. It is possible to buy external port selectors too. .

Serial ATA Tunneled Protocol

Sata 3 uses the Serial ATA Tunneled Protocol (STP) to allow SATA drives to be installed in SAS disk enclosures. They allow the SAS 6 Gbit/s HBAs to recognise SATA signals, and also send out SATA commands.

SATA connectors

A standard SATA data cable has seven conductors; 2 pairs of data lines and 3 ground lines , with 8 mm wide wafer connectors on each end. The cable length can be up to 1 metre and is typically used to connect a motherboard socket to a hard drive. A 'high end' SATA cable has the two data pairs electrically shielded to isolate the pairs and so reduce the chance of data corruption in high interference environments.

A slimline connector, intended for smaller form-factor applications like notebook optical drives was introduced in SATA 2.6. This connector has just 6 conductors.

eSATA

While SATA connectors are used to connect devices inside a computer case, eSATA or external SATA is used to go outside the computer case and connect external devices. At a 2 meter length, eSATA cables are similar to USB. eSATA was considered to be a higher standard or more professional interface compared to USB2, and was mainly used to connect external hard drives. With a transfer speed of 3Gb/s it is much faster than USB2 (480Mb/s) but is now less than USB3 (5Gb/s). However it is not quite that simple as eSATA connects devices on a one-to-one basis with the PC, so the PC's motherboard chipset only has to deal with one drive at a time, whereas USB connects multiple devices simultaneously so it several devices share the bus. Then again, you can connect multiple devices via a port multiplier.

Simple eSATA does have one drawback over USB, it just provides a data signal, not power, to external storage. eSATAp, or Power over eSATA, was designed to fix this issue, and the idea is that both eSATA and USB can be plugged into the same port and just like USB, the socket has keyed cutouts for both types of device to ensure that a connector can only be plugged in the right way. However this solution is not formally supported by either the SATA or the USB organisations.

eSATAp can supply power at +5V and +12V but the exact provision depends on the motherboard. Laptop and tablet devices do not usually provide a 12v supply, so an eSATAp port on a laptop will generally just supply +5v. Desktop devices usually supply both voltages and the port is sometimes called eSATApd with the 'd' on the end denoting dual supply. If you have an external 3.5 inch hard drive it will require a +12v supply. eSATAp on a deskstop will provide this voltage via the eSATA cable, but if you attach a 3.5 inch drive to a laptop, then you also need a separate +12v power supply to the disk.

Because an eSATAp connection is also compatible with USB, the throughput could be less than normal SATA. In theory, eSATAp ports run at a maximum of 6 Gbit/s, but actual speed will depend on the speed of the supported USB release.

eSATAp is sometimes called 'Power over eSATA', 'eSATA/USB', 'Power eSATA/USB' or 'SATA on the go'.

mSATA and µSSD

The original SATA connectors are too large for tiny portables, so mini SATA, or mSATA was designed. mSATA runs at 1.5 Gb/s or 3.0 Gb/s, about half the speed of standard SATA, and is intended for solid-state drives in mobile computing devices and thin and light laptops. It is typically about the size of a business card and uses a PCI Express Mini Card-like connector. An example product is a Samsung 840 EVO MZ-MTE1T0BW mSATA 1TB SATA III TLC Internal Solid State Drive (SSD). This device is smaller than a business card and contains 4 256GB solid state chips on a single device. Samsung states that it can perform 98,000 random read and 90,000 random write IOPS, and can achieve sequential read speeds of 540 MB/sec and write speeds of 520 MB/sec.
The main issue for mSATA is that a new SSD standard called M.2 SSD is emerging and it was developed to replace the mSATA form factor. However while m.2 SSD devices are available, this technology is still in its infancy and the demand for mSATA SSD still exists.

µSSD (micro SSD) is intended for ultra thin laptops, tablets and smartphones, where very small form factors combined with faster transfer speeds are required. A normal SATA connection uses a relatively big plug and socket, µSSD connects directly to a ball grid array (BGA) chip on a motherboard. The solid state SATA storage appliance connection is made by essentially soldering the chip direct to the motherboard.

SATA express

SATA-2 could cope with the data transfer speeds delivered by hard drives, but it hit bandwidth issues when dealing with the speeds needed for solid state drives. Even SATA-3 at 6 Gbit/s was not fast enough, so the designers tried a different approach with SATA3.2, they merged SATA with PCI express or PCIe. In very simple terms, PCIe encapsulates data into packets and each packet is striped between multiple communication channels or 'lanes'. This means that PCIe can be easily scaled up by adding more lanes. The resulting interface is called SATA express, or SATAe.

Note, SATAe is not a new version of SATA, it is just a new type of SATA connector that supports both SATA and PCIe storage devices. It connects two SATA 3.0 ports to multiple PCI Express lanes, while still being backward compatable with SATA-2. The fastest Gen3, dual lane PCIe implementations can offer up to 2GB/s of bandwidth, while those based on Gen2 PCIe will deliver at 1GB/s. Although the standard is based on PCIe, the SATAe physical interface also accommodates Serial ATA devices.

One of the main issues with any new product release is providing backward compatibility with earlier releases. Older SATA implementatons use AHCI (Advanced Host Controller Interface) and newer ones use NVMe (NVM Express), a parallel IO operation protocol especially developed for SSDs. SATAe supports both at physical (electrical and cable configuration) and logical (device driver) interface levels. However, SATA drives will only be able to use the AHCI interface, while PCIe implementations can choose between AHCI and NVM Express. As AHCI was designed with mechanical hard drives in mind it does not work well with SSDs. NVMe, on the other hand, was designed for SSDs.

At the physical plug level, various options are available with complete or limited backward compatability functionality. For example the SATA port on a motherboard is essentially two SATA ports plus an extra connector for PCIe. This means you can plug in two older SATA devices, or one PCIe device without any requirement for adapters.

There are three options available for logical device interface:

  • Legacy SATA - used for legacy SATA devices, and interfaced through the AHCI driver and legacy SATA 3.0 (6 Gbit/s) ports provided by a SATA Express controller.
  • SATA Express using AHCI - An AHCI driver enterface is used for PCI Express SSDs via PCI Express lanes. Performance is not as good as NVCMe but it is backward compatible with a wide range of existing SATA configurations in older operating systems.
  • SATA Express using NVMe - An NVMe driver interface that is used for PCI Express SSDs via PCI Express lanes. Best performance as it is optimised for SSD.

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