Disk enclosure
From Wikipedia, the free encyclopedia
A disk enclosure is essentially a specialized chassis designed to hold and power disk drives while providing a mechanism to allow them to communicate to one or more separate computers. Drive enclosures provide power to the drives therein and convert the data sent across their native data bus into a format usable by an external connection on the computers to which it is connected. In some cases, the conversion is as trivial as carrying a signal from one type of SCSI or Serial ATA cable to another. In others, it is so complicated as to require a separate embedded system that reads data over a Parallel ATA bus and retransmits it over a Universal Serial Bus, as a USB Mass Storage Device.
Contents |
[edit] Benefits
Key benefits to using external disk enclosures include:
- Adding additional storage or new removeable drive formats to small computers such as the Apple iMac, laptops, and notebooks.
- Adding more drives to any given server or workstation than their chassis can hold.
- Transferring data between non-networked computers, jokingly known as sneakernet.
- Adding a backup source with a separate power supply from the connected computer.
- Sharing the data on a drive in a network-aware enclosure.
- Preventing the heat from a disk drive from increasing the heat inside an operating computer case.
- Simple and cheap approach to hotswapping.
- Simple way of recovering the data from a broken or damaged computer.
[edit] Consumer Enclosures
In the consumer market, commonly used configurations of drive enclosures utilize magnetic hard drives or optical disk drives inside of USB, FireWire, or Serial ATA enclosures. External 3.5" floppy drive are also fairly common, following a trend to not integrate floppy drives into compact and laptop computers, started by Apple Computer with their iMac. Pre-built external drives are available through all major manufacturers of hard drives, as well as several third-parties.
As a general rule of thumb, desktop models of optical drives such as DVD-ROM drives, CD-ROM drives, and CD or DVD burners require 5.25" wide enclosure, while 3.5" floppies and desktop hard drives require a 3.5" wide enclosure. Laptop hard drives are generally 2.5" drives, but older laptops and notebooks had hard drives that varied in height, which can make it difficult to find a well-fitting chassis.
While they are less common now than they once were, it is also possible to purchase a drive chassis and mount that will convert a 3.5" hard drive into a removable hard disk that can be plugged into and removed from a mounting bracket permanently installed in a desktop PC case. The mounting bracket carries the data bus and power connections over a proprietary connector, and converts back into the drive's native data bus format and power connections inside the drive's chassis.
[edit] Enterprise Enclosures
In enterprise storage the term disk enclosure may refer to:
- The physical chassis of a more complex storage array either in a Network-attached storage or Storage area network environment.
- The entire set of components used to attach drives directly to a server. These may include a backplane, temperature sensors, enclosure management devices, and power supplies.
[edit] Connections
[edit] Native Drive Protocols
SCSI, SAS, Fibre Channel, and eSATA protocols can be used to directly connect the hard drive to an internal disk controller, without the need for any intervening controller. These native external drive protocols are extremely similar to the internal protocols, but expanded to carry power. This is the case with eSATA and the SCSI Single Connector Attachment standard. A host adapter may be necessary to connect a drive to a compatible drive controller, if it lacks an available external port.
[edit] Direct Attach Serial Protocols
USB or FireWire connections are typically used to attach consumer class external hard drives to a computer. Unlike SCSI, eSATA, or SAS these require circuitry to convert the hard disk's native signal to the appropriate protocol. Parallel ATA and internal Serial ATA hard disks are frequently connected to such chassis because nearly all computers on the market today have USB or FireWire ports, and these chassis are inexpensive and easy to find.
[edit] Network Protocols
iSCSI, NFS, or Windows File Sharing are all commonly used protocols that are used to allow an external hard drive to use a network to send data to a computer system. This type of external hard drive is also known as Network-attached storage or NAS. Often, such drives are embedded computers running operating systems such as Linux or VxWorks that use their NFS daemons and SAMBA to provide a networked file system.
