What is Hot Swap ?
| Hot Swaping Disk Drives |
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Hot swap as in Hot swapping a hard drive refers to a system designed so that the drive can be pulled from the rack, (in a server, for instance), with out cutting the power to the CPU or any other of the drives. The term "Hot Swap" refers to the common practice of either inserting, or removing SCSI disk drives in an operating bus, typically used in RAID subsystems or JBOD (just a bunch of disks) environments. The ability to "Hot Swap" a disk drive is beneficial to customers. It allows them to remove potentially defective drives from the system, or upgrade capacity without having the inconvenience and expense of taking the entire system down to replace the drive. The ANSI documents cover this function under the chapter heading "Removal and Insertion of SCSI devices". Four distinct levels of functionality are defined in Table A. The term "Hot Swap" is not actually defined in the ANSI standards, or the draft standards under development. It is interpreted as "the very restrictive Level 4 Removal and Insertion of disk drives." To avoid confusion, the two terms are linked together as "Level 4 Hot Swap." The main difference between Level 4 and the easier levels is that the bus is allowed to operate (move data or operate in any legal SCSI bus phase). Since inserting a disk into any powered bus will result in some level of electrical transients, it is necessary to insure that those transients do not interfere with, or corrupt the control of data signals present on the bus. |
| Level (1-4) | Description | Power Applied to Device | Bus State | Ground Connection to Drive | Device Circuitry Connected to Bus Pins |
| 1 | Cold Swap | No | - | - | - |
| 2 | Hot Swap while reset | Yes | Held in reset state | Must be made and maintained for 1 msec before, during, and after insertion/ removal* | - |
| 3 | Hot Swap while bus idle | Yes | Held idle (no ongoing I/O processes during insertion/removal | Same as Level 2 | Must remain glitch-free during power up or power down |
| 4 | Hot Swap on an active bus | Yes | Bus may have active I/O processes ongoing, but device being inserted or removed must be idle | Same as Level 2 | Same as Level 3 |
| *Achieved on Quantum products using SCA-2 connector | |||||
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