Choosing the Right Hard Drive

When building or selling computer systems your customers depend on you to choose the right storage solutions.  Choosing the right hard drives is very critical because it affects both system performance and the integrity of the user’s valuable data.  Here is an overview of the most important technologies available today.

Hard Drive Interfaces

PATA (Parallel ATA) – This is the standard IDE parallel data interface that has been used on PCs for almost 20 years and which has seen some improvements recently with the move up to ATA133 (133 Megahertz) bus speeds with the help of 80-line ribbon cables.  Even with the ultra cables PATA technology suffers from the inability to go to higher bus speeds due to problems with high-frequency cross-talk.

SATA (Serial ATA) – Serial ATA is the next generation IDE communication interface and uses a single device per cable.  The technology draws its name from the way it transmits signals in a single stream (serially) compared with the multiple streams found in parallel technology. The current generation SATA drives are version 1.0 and work at 150MB/sec, support hot-swap, plug and play, and requires no jumpers.  Each SATA drive is attached to the controller using a single thin cable so devices do not share the bus.  SATA specification 2.0 (to be released over the next couple years) will run at speeds of 300MB/sec  plus add additional advanced features to the ones in the SATA 1.0 specification.  For more info go to this page: Serial ATA (IDE) FAQ.s

SCSI (Small Computer System Interface) – The SCSI standard has greatly evolved over time to the current Ultra320 standard that allows up to 15 drives on a single channel using 320MB/sec bus speeds.  SCSI is typically very fast and a perfect solution for server systems that need a large amount of very reliable storage.  Current drives SCSI use 68-pin connectors, but hot-swap SCSI drives use a special 80-pin connector called SCA (Single Connector Attachment).  Hot swapping allows you to change the drive with the system power on.

SAS (Serial Attached SCSI) – SAS is the next evolution of SCSI beyond Ultra320.  It leverages the power of serial communication bus/protocol like SATA to increase performance.  SAS is not yet on the market but will offer many features not found in today's mainstream storage solutions. These include drive addressability of up to 16,256 devices per port, plus reliable point-to-point serial connections at speeds of up to 3G bps.  SAS drives will also use the same internal data and power interfaces as SATA.  In addition, the similar SAS and SATA physical interfaces enable a new universal SAS backplane that provides connectivity to both SAS drives and SATA drives. This eliminates the need for separate SCSI and ATA drive backplanes.  For more info see: http://www.scsita.org/aboutscsi/sas/tutorials.html 

External Hard disks (USB/Firewire/SATA)

With USB 2.0 controllers now standard on all motherboard it is easy to connect external drives that can communicate at high speeds.  Most such solutions provide a hard drive enclosure that will hold an IDE PATA or SATA drive and that has its own power connector.  The external enclosure then connects to the system using USB 2.0 or Firewire, or optionally to a SATA card with external SATA cable (such as the Highpoint RocketRAID 1542).  External drives may support booting in some cases.

ATA (IDE) Chart:

SCSI Interface Chart:

Hard Drive Performance Factors

There are a number of factors that determine a hard drive’s performance, which in itself has a great impact on overall system performance.  The faster data can be read from or written to the hard disk the faster applications will load, data can be stored, and virtual memory (i.e. the swap file) can function.  This speed is called the access time. IDE drives typically have access times of 8.0 - 9.5 milliseconds, while SCSI has average 3.9 - 5.2 milliseconds access time.  The main factors are :

RPM (Rotational Speed): The faster the disk can spin the faster the R/W (Read/Write) head can get to the right spot on the disk.  For IDE 5400RPM speeds are giving way to 7200RPM.  Many newer SCSI drives spin at 10,000-15,000RPM, giving them a definite edge.   Higher RPM speeds can generate more heat so extra cooling and design considerations will come into play when using multiple drives.

RAID Technologies

One of the most effective ways to increase the reliability and/or performance of your storage system is to use RAID.  RAID can be used in personal computers, workstations, servers, or rackmount units using either IDE (PATA or SATA) or SCSI drives.  IDE RAID allows for larger capacity RAID arrays than SCSI, but at lower performance.

The best way to protect your valuable data on a budget is IDE RAID LEVEL 1.  This is also known as "mirroring" where the data from one drive is copied exactly onto another drive. If one hard drive fails, you have a perfect duplicate to use.  This is also good for entry level servers that must be kept running 24 hours a day.

The best overall RAID for building mission-critical servers is RAID LEVEL 5, which involves data striping + parity information that is spread across a minimum of three hard disks.  With RAID 5 drive you a combination of good performance and excellent data protection.  Some servers will use RAID 1 array for the OS and a second RAID 5 array for data.

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