Dell™ PowerEdge™ Expandable RAID Controller 4/Di User's Guide
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An array of disk drives combines the storage space on the disk drives into a single segment of contiguous storage space. The PERC 4/Di can group disk drives on one or more SCSI channels into an array. A hot spare drive does not actively participate in an array.
Software that provides common control and management for a disk array. Array Management Software most often executes in a disk adapter or intelligent host bus adapter. When it executes in a disk adapter or adapter, Array Management Software is often called firmware.
Array spanning by a logical drive combines storage space in two arrays of disk drives into a single, contiguous storage space in a logical drive. PERC 4/Di logical drives can span consecutively numbered arrays that each consist of the same number of disk drives. Array spanning promotes RAID level 1 to RAID level 10 and RAID level 5 to RAID level 50.
Operations that bear no relationship to each other in time and can overlap. The concept of asynchronous I/O operations is central to independent access arrays in throughput-intensive applications.
(Basic Input/Output System) The part of the operating system in an IBM PC-compatible computer that provides the lowest level interface to peripheral devices. The BIOS is stored in ROM in every IBM or compatible PC. BIOS also refers to the Basic Input/Input Output System of other "intelligent" devices, such as RAID controllers.
Specifies that reads are buffered in cache memory, but does not override the other cache policies, such as read ahead or write.
The process of utilizing a high speed memory buffer, referred to as a "cache", in order to speed up the overall read or write performance. This cache can be accessed at a higher speed than a disk subsystem. To improve read performance, the cache usually contains the most recently accessed data, as well as data from adjacent disk sectors. To improve write performance, the cache may temporarily store data in accordance with its write back policies. See the definition of Write-Back for more information.
An electrical path for the transfer of data and control information between a disk and a disk adapter. A channel can also be referred to as a "bus", such as a SCSI bus.
The replacement or exchange of a device in a computer system after powering down the system. In reference to disk subsystems, a cold swap requires that you turn the power off before replacing a defective hard drive.
An examination of the disk system to determine if all conditions are valid for the specified configuration (such as parity).
The amount of data per unit time moved through a channel. For disk I/O, bandwidth is expressed in megabytes per second (MB/s).
A disk drive that has become non-functional or has decreased in performance.
Specifies that reads are not buffered in cache memory, but does not override the other cache policies, such as read ahead or write.
A non-volatile, randomly addressable, rewriteable mass storage device, including both rotating magnetic and optical storage devices and solid-state storage devices, or non-volatile electronic storage elements.
A collection of disks from one or more disk subsystems controlled by array management software. The array management software controls the disks and presents them to the array operating environment as a virtual disk.
A variation on disk mirroring in which a second disk adapter or host adapter and redundant disk drives are present.
Writing duplicate data to more than one (usually two) hard disks to protect against data loss in the event of device failure. Disk mirroring is a common feature of RAID systems.
The process of creating one logical drive composed of multiple arrays. Spanning is used to create complex RAID sets, such as RAID levels 10 and 50. Spanning utilizes striping to distribute data across all member disk drives.
A type of disk array mapping. Consecutive stripes of data are mapped round-robin to consecutive array members. A striped array (RAID level 0) provides high I/O performance at low cost, but provides less data reliability than any member disk.
A collection of disks and the hardware that controls them and connects them to one or more controllers. The hardware can include an intelligent adapter, or the disks can attach directly to a system I/O bus adapter.
A technique that achieves maximum data transfer bandwidth by constantly keeping two I/O requests outstanding for adjacent data. A software component begins a double-buffered I/O stream by issuing two requests in rapid sequence. Thereafter, each time an I/O request completes, another is immediately issued. If the disk subsystem can process requests fast enough, double buffering allows data to be transferred at the full-volume transfer rate.
A drive that has ceased to function, that consistently functions improperly, or that is inaccessible.
A variant on the SCSI-2 bus. It uses the same 8-bit bus as the original SCSI-1 but runs at up to 10MBs (double the speed of SCSI-1).
Software stored in read-only memory (ROM) or Programmable ROM (PROM). Firmware is often responsible for the behavior of a system when it is first turned on. A typical example would be a monitor program in a computer that loads the full operating system from disk or from a network and then passes control to the operating system.
The process of writing a specific value to all data fields on a physical drive, (hard drive), to map out unreadable or bad sectors. Because most hard drives are formatted when manufactured, formatting is usually done only if a hard disk generates many media errors.
A gigabyte; 1,000,000,000 (10 to the ninth power) bytes.
Any computer to which disks are directly attached, (not attached remotely.) Mainframes, workstations, and personal computers can all be considered host computers.
An idle, powered on, stand-by drive ready for immediate use in case of disk failure. It does not contain any user data. Up to eight disk drives can be assigned as hot spares for an adapter. A hot spare can be dedicated to a single redundant array or it can be part of the global hot-spare pool for all arrays controlled by the adapter.
When a disk fails, the controllers' firmware automatically replaces and rebuilds the data from the failed drive to the hot spare. Data can be rebuilt only from logical drives with redundancy (RAID levels 1, 5, 10, or 50; not RAID 0), and the hot spare must have sufficient capacity. The system administrator can replace the failed disk drive and designate the replacement disk drive as a new hot spare.
The manual replacement of a failed drive while the disk subsystem is running (performing its normal functions).
Hot swap drives allow a system administrator to replace a failed disk drive in a system without powering down the system and suspending services. The hot swap drive is pulled from its slot in the drive cage; all power and cabling connections are integrated into the drive enclosure backplane. The replacement hot swap drive can then slide into the slot. Hot swapping only works for RAID 1, 5, and 10 configurations.
The process of writing zeros to the data fields of a logical drive and, in fault tolerant RAID levels, generating the corresponding parity to put the logical drive in a Ready state. Initializing erases previous data and generates parity so that the logical drive will pass a consistency check. Arrays will work without initializing, but they can fail a consistency check because the parity fields have not been generated.
A host system software component (usually part of the operating system) that controls the operation of peripheral adapters attached to the host system. I/O drivers communicate between applications and I/O devices and in some cases participate in data transfers.
A complete or partial representation of a logical array. The storage space in a logical drive is spread across all the physical drives in the array or spanned arrays. Each PERC 4/Di adapter can be configured with up to forty logical drives in any combination of sizes. Configure at least one logical drive for each array. A logical drive can be in one of three states:
I/O operations can be performed only with logical drives that are online or degraded.
The relation between multiple data addressing schemes, especially conversions between member disk block addresses and block addresses of the virtual disks presented to the operating environment by array management software.
A megabyte; an abbreviation for 1,000,000 (10 to the sixth power) bytes.
The process of providing complete redundancy using two disk drives, by maintaining an exact copy of one disk drive's data on the second disk drive. If one disk drive fails, the contents of the other disk drive can be used to maintain the integrity of the system and to reconstruct the failed drives.
Having multiple concurrent or pseudo-concurrent execution sequences. Multi-threaded processes allow throughput-intensive applications to efficiently use resources to increase I/O performance.
A nanosecond, 10^-9 second.
An online device is a device that is accessible.
Capacity expansion by adding volume or another hard drive, while the host system is accessible and/or active.
An operating environment can include the host system where an array is attached, any I/O buses and adapters, the host operating system and any additional software required to operate the array. For host-based arrays, the operating environment includes I/O driver software for the member disks but does not include array management software, which is regarded as part of the array itself.
An extra bit added to a byte or word to reveal errors in storage (in RAM or disk) or transmission. Parity is used to generate a set of redundancy data from two or more parent data sets. The redundancy data can be used to reconstruct one of the parent data sets. However, parity data does not fully duplicate the parent data sets. In RAID, this method is applied to entire drives or stripes across all disk drives in an array. Parity consists of dedicated parity, in which the parity of the data on two or more drives is stored on an additional drive, and distributed parity, in which the parity data are distributed among all the drives in the system. If a single drive fails, it can be rebuilt from the parity of the applicable data on the remaining drives.
A complete or partial representation of a logical drive, usually represented to a user by an operating system as a physical disk. Also called a logical volume.
The Dell™ PERC 4/Di consists of an LSI 1030 chip on the motherboard to offer RAID control capabilities. PERC 4/Di supports all dual-ended and low-voltage differential (LVD) SCSI devices on Ultra3 and Wide SCSI channels with data transfer rates up to 320 MB/s (Megabytes per second).
PERC 4/Di provides reliability, high performance, and fault-tolerant disk subsystem management. It is an ideal RAID solution for the internal storage of Dell's workgroup, departmental, and enterprise systems. PERC 4/Di offers a cost-effective way to implement RAID in a server and provides reliability, high performance, and fault-tolerant disk subsystem management.
A hard drive. A hard drive consists of one or more rigid magnetic discs rotating about a central axle, with associated read/write heads and electronics. A physical disk is used to store information, (data), in a non-volatile and randomly accessible memory space.
The ability of adapters to detect when hard drives have been moved to different slots in the storage enclosure, such as after a hot swap.
A set of formal rules describing how to transmit data, generally across a network or when communicating with storage sub-systems. Low-level protocols define the electrical and physical standards to be observed, bit- and byte-ordering, and the transmission and error detection and correction of the bit stream. High-level protocols deal with the data formatting, including the message syntax, the terminal to computer dialogue, character sets, sequencing of messages, etc.
Redundant Array of Independent Disks (originally Redundant Array of Inexpensive Disks) is an array of multiple small, independent hard drives that yields performance exceeding that of a Single Large Expensive Disk (SLED). A RAID disk subsystem can improve I/O performance relative to a system using only a single drive. The RAID array appears to the controller as a single storage unit. I/O is expedited because several disks can be accessed simultaneously. Redundant RAID levels (RAID levels 1, 5, 10, and 50), provide data protection.
A style of redundancy applied to an array. It can increase the performance of a logical drive though it may decrease usable capacity. Each logical array must have a RAID level assigned to it.
A memory caching capability in some adapters that allows them to read sequentially ahead of requested data and store the additional data in cache memory, anticipating that the additional data will be needed soon. Read-Ahead supplies sequential data faster, but is not as effective when accessing random data.
A condition in which a workable hard drive is neither online nor a hot spare and is available to add to an array or to designate as a hot spare.
The regeneration of all data to a replacement disk from a failed disk in a logical drive with a RAID level 1, 5, 10 or 50 array. A disk rebuild normally occurs without interrupting normal operations on the affected logical drive, though some degradation of performance of the disk subsystem can occur.
The percentage of CPU resources devoted to rebuilding.
The act of remaking a logical drive after changing RAID levels or adding a physical drive to an existing array.
The provision of multiple interchangeable components to perform a single function to cope with failures and errors. Common forms of hardware redundancy are disk mirroring, implementations of parity disks or distributed parity.
A hard drive replacing a failed member disk in a RAID array.
A component or collection of components in a system or subsystem that is always replaced as a unit when any part of the collection fails. Typical replacement units in a disk subsystem include disks, adapter logic boards, power supplies and cables.
(Small Computer System Interface) A processor-independent standard for system-level interfacing between a computer and intelligent devices, such as hard disks, floppy disks, CD-ROM, printers, and scanners. SCSI can connect up to 15 devices to a single adapter (or host adapter) on the computer's bus. SCSI transfers 8, 16 or 32 bits in parallel and can operate in either asynchronous or synchronous modes. The synchronous transfer rate is up to 320 MB/s.
The original standard is now called SCSI-1 to distinguish it from SCSI-2 and SCSI-3, which include specifications of Wide SCSI (a 16-bit bus) and Fast SCSI (10 MB/s transfer.) Ultra 160M SCSI is a subset of Ultra3 SCSI and allows a maximum throughput of 160 MB/s, which is twice as fast as Wide Ultra2 SCSI. Ultra320 allows a maximum throughput of 320 MB/s.
PERC 4/Di controls hard drives via 320M SCSI buses (channels) over which the system transfers data in either LVD or 320M SCSI modes. Each adapter controls two SCSI channels.
A SCSI disk drive (physical drive) can be in one of these four states:
Each SCSI device on an PERC 4/Di SCSI bus must have a different SCSI address number (Target ID or TID) from 0 to 15. Notice that one ID is used by the SCSI controller, usually ID 7. Set the SCSI ID switch on each disk drive to the correct SCSI address. See the PERC 4/Di documentation, chassis labels or disk enclosure documentation for the correct switch settings.
A hard drive available to back up the data of other drives.
Segmentation of logically sequential data, such as a single file, so that segments can be written to multiple physical devices in a round-robin fashion. This technique is useful if the processor can read or write data faster than a single disk can supply or accept it. While data is being transferred from the first disk, the second disk can locate the next segment. Data striping is used in some modern databases and in certain RAID devices.
The amount of data contiguously written to each disk. Also called "stripe depth." You can specify stripe sizes of 4 KB, 8 KB, 16 KB, 32 KB, 64 KB and 128 KB for each logical drive. A larger stripe size produces improved read performance, especially if most of the reads are sequential. For mostly random reads, select a smaller stripe size.
The number of disk drives across which the data are striped.
A resistor connected to a signal wire in a bus or network for impedance matching to prevent reflections, e.g., a resistor connected across signal wires at the end of a SCSI cable.
A variant on the SCSI-2 interface. Wide SCSI uses a 16-bit bus, double the width of the original SCSI-1.
In Write Back caching mode, the controller sends a data transfer completion signal to the host when the controller cache has received all the data in a disk write transaction. Data are written to the disk subsystem in accordance with policies set up by the controller. These policies include the amount of dirty/clean cache lines, the number of cache lines available, elapsed time from the last cache flush, and others.
In Write Through caching mode, the controller sends a data transfer completion signal to the host when the disk subsystem has received all the data in a transaction. The controller cache is not used.
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