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RAID Array & Server Glossary of Computer Terms (Letter S)

February 19th, 2009

SCSI
Small computer system interface (pronounced scuzzy). The fast, intelligent input/output parallel bus used by high-performance peripherals.

Software-based array
An array in which all management functions including parity calculation (XOR) are performed by the host server CPU. These products are low priced but have high CPU utilization and limited fault-tolerant features. High-performance, low-cost array adapters are quickly replacing these inferior software-based arrays.

System disk
The disk (or array) on which a system’s operating system is stored and from which it is initially loaded into system memory.

SAF-TE
SCSI Accessed Fault-Tolerant Enclosure, an “open” specification designed to provide a comprehensive standardized method to monitor and report status information on the condition of disk drives, power supplies, and cooling systems used in high availability LAN servers and storage subsystems. The specification is independent of hardware I/O cabling, operating systems, server platforms, and RAID implementation because the enclosure itself is treated as simply another device on the SCSI bus. Many other leading server, storage, and RAID controller manufacturers worldwide have endorsed the SAF-TE specification. Products compliant with the SAF-TE specification will reduce the cost of managing storage enclosures, making it easier for a LAN administrator to obtain base-level fault-tolerant alert notification and status information. All Mylex RAID controllers feature SAF-TE.

Sector
The unit in which data is physically stored and protected against errors on a fixed-block architecture disk.

Segment Size
See Cache Line Size

Sequential I/O
A type of read and write operation where entire blocks of data are accessed one after another in sequence, as opposed to randomly.

SES
SCSI Enclosure Services, a standard for SCSI access to services within an enclosure containing one or more SCSI devices. For disk drives, power supplies, cooling elements, and temperature sensors, the actions performed are the same as for SAF-TE monitoring. If a UPS is connected to any SES-monitored enclosures, and an AC failure or two minute warning is reported, conservative cache is enabled and all system drives are switched to write-through cache. Primarily used in fibre enclosures.

Session
The period of time between any two consecutive system shutdowns; system shutdown may be either a power off/on, or a hardware reset.

SMART
Self-Monitoring Analysis and Reporting Technology, the industry standard reliability prediction indicator for both the ATA/IDE (advanced technology attachment/integrated drive electronics) and SCSI hard disk drives. Hard disk drives with SMART offer early warning of some hard disk failures so critical data can be protected.

Spanning
A process that provides the ability to configure multiple drive packs or parts of multiple drive packs. In effect, spanning allows the volume used for data processing to be larger than a single drive. Spanning increases I/O speeds, however, the probability of drive failure increases as more drives are added to a drive pack. Spanned drive packs use striping for data processing. See also Striping and Drive Groups, Drive Packs.

Standard Disk Drive
This term refers to a hard disk drive with SCSI, IDE, or other interface, attached to the host system through a standard disk controller.

Standby Replacement of Disks
See also Hot Spare. One of the most important features the RAID controller provides to achieve automatic, non-stop service with a high degree of fault-tolerance. The controller automatically carries out the rebuild operation when a SCSI disk drive fails and both of the following conditions are true:

  • A “standby” SCSI disk drive of identical size is found attached to the same controller;
  • All of the system drives that are dependent on the failed disk are redundant system drives, e.g., RAID 1, RAID 3, RAID 5, and RAID 0+1.

Note: The standby rebuild will only happen on the same DAC960 controller, never across DAC960 controllers.

During the automatic rebuild process, system activity continues as normal. System performance may degrade slightly during the rebuild process.

To use the standby rebuild feature, you should always maintain a standby SCSI disk in your system. When a disk fails, the standby disk will automatically replace the failed drive and the data will be rebuilt. The system administrator can disconnect and remove the bad disk and replace it with a new disk. The administrator can then make this new disk a standby.

The standby replacement table has a limit of 8 automatic replacements in any session (from power-on/reset to the next power-off/reset). When the limit of 8 is reached and a disk failure occurs, the standby replacement will occur but will not be recorded in the replacement table.

To clear the “standby replacement” table, reboot the system from a DOS bootable floppy, run the configuration utility and select the option ‘view/update configuration’ from the main menu. A red box labeled ‘Drive Remap List’ will be displayed. Selecting the box will allow you to continue. You should save the configuration without making any changes, and exit the configuration utility. This will clear the replacement table. You may now proceed to boot your system and continue normal operations.

In normal use, the replacement table limit of 8 should not cause any problems. Assuming that a disk fails about once a year (drives we support generally come with a 5-year warranty), the system would run continuously for a minimum of 8 years before the table would need to be cleared.

Storage Device
A collective term for disks, tape transports, and other mechanisms capable of non-volatile data storage.

Stripe Order
The order in which SCSI disk drives appear within a drive group. This order must be maintained, and is critical to the controller’s ability to “rebuild” failed drives.

Stripe Size
The size, in kilobytes (1024 bytes) of a single I/O operation. A stripe of data (data residing in actual physical disk sectors, which are logically ordered first to last) is divided over all disks in the drive group.

Stripe Width
The number of striped SCSI drives within a drive group.

Striping
The storing of a sequential block of incoming data across multiple SCSI drives in a group. For example, if there are 3 SCSI drives in a group, the data will be separated into blocks. Block 1 of the data will be stored on SCSI drive 1, block 2 on SCSI drive 2, block 3 on SCSI drive 3, block 4 on SCSI drive 1, block 5 on SCSI drive 2, and so on. This storage method increases the disk system throughput by ensuring a balanced load among all drives.

Sub-System Storage
A collection of disks providing data storage space to a system user.

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