Guides

• How do they work?
• Capacity
• Interface
• PATA
• SATA
• SCSI
• USB 2.0
• Firewire
• Power
• Form factors
• Internal or external?
• Other hard disk specifications
• RPM
• Seek/access time
• Native command queuing
• MTBF/MTTF
• Noise
• Using the hard disk
• Multiple drives
• RAID

Using the hard disk

Hard disk performance and getting the right drive for you

The ultimate performance of a hard disk is determined by many factors, including some that are unknowable by the end user. The interface, the rpm and the seek times are the best indicators that we, as consumers, have of the speed of a hard disk. Several X factors also apply: the quality of the drive electronics, the density of the magnetic zones (the "areal density" - the more dense the zones, the more data is carried past the head with each revolution), the number of platters and so forth. It's often hard to tell how fast a drive is without proper benchmarking, so we recommend checking out Web reviews of hard disks.

Buying the correct drive for you is largely a matter of whether you value speed or capacity. One thing that long-time computer users will tell you is that you can never have too much space, so for most people getting the highest capacity at the lowest price is the biggest concern.

If, however, your goal is not to archive vast amounts of information, but to get the highest speeds from your PC, you might look to faster (but lower capacity) drives, such as 10,000rpm drives.

Even if you're not a speed freak, don't ignore this aspect of the hard disk. The hard disk is the part of your computer that you're most often waiting on, whether it be to open a new program, access virtual memory or load a new level in the game. A hard disk that's a little faster than the others can make a big difference to your computing experience.

Multiple drives

One thing you can do to optimise your system is use multiple drives, possibly in a RAID configuration (see below).

A PC can have as many hard disks as its interfaces and case can afford. The easiest way to increase the storage capacity of your system is to add a new hard disk (you can usually keep you old hard disks as well, unless you run out of room).

If you do plan to have more than one hard disk, there's a simple rule to follow - make the fastest one your system disk. This is the disk that has the operating system, stores the virtual memory and contains installed applications. It's the disk that is accessed most often, and because the head has to move around a lot, the access time is particularly crucial on this disk. It's also a good idea to keep this disk as free from clutter as possible and defragment it often (for reasons we'll explain below).

RAID

If you do decide to get multiple drives, you can look to a RAID (Redundant Array of Inexpensive Disks) configuration. Many consumer SATA motherboards now support RAID 0 and 1, along with what are called JBOD (just a bunch of disks) configurations. RAID is entirely a function of the disk controller. A hard disk does not need to actively "support" RAID - any disk can be placed in a RAID.

The basic principal of RAID is to arrange a group of drives into what appear to the operating system as a single drive. With the drives working together, a RAID array can improve a system's performance and data reliability.

RAID 0, called striping, is where part of a file is stored on each disk. When the data is read, it is read in parallel from the disks in the RAID. Because each disk has to do less work, the overall speed of the file transfer is greater - RAID 0 can give you significant performance improvements. With RAID 0, the disks should to be similar is size - the total capacity of the array will be dependent on the smallest drive in the array.

The big danger of RAID 0 is that if one drive dies, then all the data in the array is lost. For this reason, RAID 0 is rarely used for vital information.

RAID 1, or mirroring, replicates all the data across all the drives in the array. This gives you reliability and speed (since it still reads data in parallel off multiple drives), but reduces your capacity. Say you had two 200GB drives, and set them up in a RAID 1. The total capacity of the array is only 200GB (not 400GB), because all the data on drive one is replicated on drive two. If one of the drives dies, then no data is lost (because all the data is still on the other). RAID 1 provides speed and reliability, at a huge cost in capacity.

There are other RAID formats - 2, 3, 4 and 5 - with various levels of functionality, but these are rarely available without an expensive RAID expansion card, and are generally not seen in consumer PCs.

JBODS are simply a way of agglomerating multiple hard drives into a singe logical drive. If you had a 10GB, 80GB and 200GB drive, you could make a JBOD of 290GB, appearing to the operating system as a single drive. They provide no performance or reliability benefits.

File systems, fragmentation and slow downs

A hard disk merely provides a way of storing data, it doesn't tell the PC how to organise it. That's the job of the file system.

A file system is essentially a directory of the data stored on a hard disk. It's the PCs way of storing information on where files are physically located on the disk - for instance, that the file "fluffythecat.jpg" is located on track 31, sector 18 of the hard disk.

Different file systems have different performance levels, and many quirks besides, such as the capacity of the hard disk they can support, support for encryption and how the geometry of the hard disk is divided up into addressable regions.

One of the most important aspects of a file system is how it deals with fragmentation. Newer file systems, like NTFS, tend to deal with fragmentation much better.

Fragmentation occurs when a file tries to fit into a gap on the hard disk that is too small for it. Say you delete a 10Kb file, which leaves a gap free on the hard disk. Then you try and write a 15Kb file. Only the first 10Kb can fit where the old file went. The rest has to go elsewhere on the disk, thus fragmenting the file.

Fragmentation will require that the head jump from one part of the disk to another in order to complete the file reading. If the head has to move around the hard disk hither and thither, a lot of time is going to be wasted (this is why access times are so important). For this reason, defragging your disk periodically is important. Defragging will attempt to re-align files so that the entire file can be found in one place. In Windows XP, the defragger can be found under Start-Programs-Accessories-System Tools.

One other thing that users often find is that hard disks get slower as they fill up. There is a very good reason for this: shorter tracks. Hard disks write first to the outside tracks of the platter, and then work their way in towards the centre. The outside tracks have a larger circumference than the inner tracks, and therefore each revolution of the hard disk covers more area on the outer tracks than it does on the inner (and thus passes more data to the read head). As the disk fills up, more data is being written to the slower inner tracks, and so you'll see an overall decrease in speed on that data. There's no real solution to this problem, except to keep your PC as free from clutter and excess data as possible. Defragging will move all the data to the outer tracks.