About Hard Drives
Some notes from aaaaages ago, not necessarily part of our syllabus.
Wikipedia defines a Hard Drive as: "A hard disk (or "hard disc" or "hard drive" or "hard disk drive") is a computer storage device that stores data on rotating magnetic surfaces." Now, I'm less interested in the ins and outs of how the hard drive works and more in the types of hard drives, and their practical applications.
Hard Drive Controllers
Hard drives connect to the motherboard via a controller, this might be integrated in the motherboard (as is usually the case with ATA), or in the form of an additional card in the computer (as is usually the case with SCSI). The controller will have a number of channels, or buses. These are physical connections on the controller that link to the drives. One channel may support a number of hard drives depending on the configuration and type of drive.
Hard Drive Interfaces
There are a number of types of hard drive, these types have different characteristics and connect to the rest of the computer with different interfaces. Primarily there are four we need to be concerned with at the moment.
- (P)ATA (Parallel Advanced Technology Attachment) is the type most often originally used in desktop computers (although this is changing over time).
- Virtually all computers will have support for ATA devices.
- The same type of attachment is commonly used to connect CD and DVD drives as well as hard drives.
- Typically a motherboard will have two IDE channels, each capable of supporting two ATA devices.
- Where there are two devices on a channel (or cable), one must be set as master and one as slave. This is just to identify them to the computer, these may be referred to (more accurately) on the BIOS as device0 (master) and device1 (slave). You can change the master/slave setting on a hard drive using a jumper on the back.
- SATA (Serial Advanced Technology Attachment) is intended as a replacement for ATA drives. Whilst it has not been around long, it is often now used in high performance desktops or low performance servers.
- Many newer computers will have support for
SATA devices, but support can be added on older machines with a SATA controller card. - One SATA channel can only support one SATA device. Generally only hard drives are available in SATA.
- Two varieties of SATA exist, SATA-I and SATA-II (its replacement). SATA-II is twice as fast, and newer SATA-II drives can still be used with a SATA-I controller so long as the drives are set to SATA-I mode, rather than SATA-II mode. This is generally done via a jumper on the back of the drive.
- Many newer computers will have support for
- SCSI (Small Computer System Interface) drives are commonly used in servers due to their increased reliability and performance compared to their rival ATA drives.
- Most machines will not support SCSI drives by default, a controller card will have to be
installed.
- SCSI can connect any type of device, not just hard drive, but is most often used with hard drives and tape drives.
- One SCSI channel can support a number of SCSI devices, commonly 6, or 16 depending on the exact SCSI version being used. Each device on the channel must be assigned a unique number (either by a jumper or similar on the back of the disk) to identify itself with.
- Most machines will not support SCSI drives by default, a controller card will have to be
- SAS (Serial Attached SCSI). These have similar characteristics to SCSI drives, but communicate in serial rather than parallel.
- Most machines will not support SAS drives by default, a controller card will have to be installed.
- One SAS channel can support a number of devices, these can include backwards compatibility with SATA devices.
- SAS is intended as a replacement for SCSI due to its faster performance.
That's quite a lot to take in. In general it is the case that desktops tend to use ATA drives, which are now being replaced by SATA drives. Servers on the other hand have tended to use SCSI drives, and are now starting to use SAS drives. SAS is viewed as a convergence of SCSI and SATA, that may one day replace all hard drives. Of course there are exceptions to these general rules, there are other factors that have to be taken into consideration.
Summary Table
| ATA | SATA | SCSI | SAS |
|---|---|---|---|
| Compatible with virtually all computers. | May require a controller card for compatibility with older machines. | Compatible with many server machines, otherwise a controller card is required. | Compatible with newer server machines, otherwise a controller card is required. |
| RAID is not possible at hardware level. | RAID is possible with some controller cards at hardware level. | RAID is generally possible at hardware level. | RAID is virtually always possible at hardware level. |
| Can be used for CD, DVD, Floppy and hard drives. | Generally only used for hard drives, although CD drives are available. | Generally used for hard drives and tape drives, although compatibility with lots of devices is possible. | Generally used for hard drives, although compatibility with other devices is possible. Backwards compatible with SATA devices. |
| Speeds of 80MB/s theoretically possible. | Speeds of 150 MB/s for SATA-I and 300MB/s for SATA-II possible. | Speeds of upto 320MB/s per channel possible. | Speeds of upto 600 MB/s possible per device. |
| Supports upto two devices per channel. | Supports only one device per channel. | Supports upto 16 devices per channel. | Supports thousands of devices per channel if the correct expanders are used. |
| Tend to be very cheap. | Prices have dropped a lot, this is very cost effective storage. | Large disks are still very expensive. | Large disks are very expensive. |