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Apricot and RAID RAID was originally an acronym for Redundant Array of Inexpensive Disks. Each RAID level describes the method in which data is distributed across a number of disk drives, known as an array. It must be noted however, that these are not true "levels", as the higher levels don't contain all the functions of the lower levels, and RAID 5 is not necessarily better than RAID 3. The birth of drive arrays effectively started with a paper published by Berkeley University (CA), this paper classified the different architectures as RAID levels. The driving force was to provide minicomputers with high performance drive subsystems using the (then) newly available low cost 5.25" Winchesters rather than the very expensive mass storage magnetic drives used by mainframes. The concept was to provide the required capacity using multiple 5.25" drives connected as an array; in effect the drives were operated in parallel to increase their performance. The word 'Inexpensive' in RAID's original definition is now somewhat of a misnomer. The drives used in a RAID solution tend to be high quality, fast, SCSI devices which are by no means the cheapest drives available for PCs. Consequently, the definition of RAID is undergoing a change from Redundant Array of Inexpensive Disks to Redundant Array of Independent Disks.
The original Berkeley paper described five RAID levels. Of these only RAID 1, 3 and 5 are now considered practical solutions. RAID 2 and 4 have been superseded by RAID 3 and 5 respectively as they offer similar or superior performance at no additional cost. There are other levels of RAID, but of these, only RAID 0 is accepted as an industry standard. Note: For the purpose of this document, the diagrams illustrate a RAID solution using either two or four disk drives - this is not a mandatory requirement. Each RAID solution should be tailored to the customer's application using the appropriate number of drives. RAID LEVEL 0 RAID 0 was not described by the Berkeley team but due to its wide availability has since been endorsed by the RAID Advisory Board (RAB). RAB is an association of suppliers and consumers of RAID related products and other organisations with an interest in RAID technology set up to standardise RAID related terminology throughout the industry.
RAID 0, also known as disk striping, works by segmenting a user file into 'data blocks' and then writing the blocks sequentially across the drives in the array. Disk striping offers excellent performance by transferring data using several disks at once. However, in this configuration only one copy of the data is stored, and therefore no fault tolerance is achieved. In fact this method is even more open to data loss; when four drives make up a single logical drive, that logical drive is four times more likely to fail than a single physical disk. Strictly speaking RAID level 0 is not a RAID at all as there is no redundancy. RAID 0 can be implemented on any NetWare or Windows NT system with more than 1 hard disk, without the introduction of any special hardware. Under NetWare this option is called “Spanning a volume” and is achieved when creating a new volume either at installation time or when adding another drive. Under Windows NT 3.5 this is a standard feature of the Disk Administrator and is called “Create Stripe Set”.
RAID LEVEL 1 RAID 1 is also known as mirroring or shadowing. In its simplest form two disks are connected to a single controller, and as data is written to one disk the same information is also written to the mirrored disk.
If one drive fails, the system continues to run using data from the remaining drive. However, if the controller fails, both drives fail and the system is down. To overcome this, two controller cards can be used, which is a method known as duplexing. The biggest disadvantage of mirroring is its cost. With each active drive duplicated exactly, there is 50 percent data redundancy and twice the cost of the same system with no mirroring. Again, RAID 1 can be implemented on any NetWare or Windows NT system with more than 1 hard disk, without the introduction of any special hardware.
RAID LEVEL 3 RAID 3 employs a technique called 'disk striping with redundancy'. This offers the advantage of disk striping but without the very high level of data redundancy and cost of mirroring.
It provides data redundancy by producing 'parity' data that is stored on a dedicated drive referred to as the parity drive. Parity data is created by applying complex mathematical calculations to the original data which extract relevant bits of information that can be stored using a fraction of the space used by a mirror image. In the event of a drive failure, this parity information can be used to reconstruct the original data. The disadvantage with this technique is that every time a write occurs, parity information must be stored to a single disk. The idea of introducing “Striping” is so that when a system is under heavy load, the writes are spread across a number of drives in parallel, implementing RAID 3 therefore reduces the performance of the system by having only a single drive for parity.
RAID LEVEL 5 RAID 5 combines data striping and the storing of parity data on all drives, as opposed to one dedicated parity drive. This option requires a minimum of three hard disk drives.
RAID 5 and RAID 3 are very similar in terms of the benefits they offer, their cost implications and the way in which they function. However, there are some fundamental differences that make them suitable for very different applications. The process of storing parity data across a number of drives, rather than one dedicated parity drive, alleviates the bottleneck created by many parity write requests being directed to one drive. In a RAID 5 configuration the amount of data actually stored on each individual drive during the data striping process is larger than in RAID 3. Consequently requests are typically serviced from one drive, as opposed to several drives, and thus the potential competition for access to one drive is minimised. Due to this process RAID 5 is particularly effective for applications which access a high number of small files such as large databases and transaction processing systems (which most network users will be running). In contrast, RAID 3 is particularly effective for applications such as image processing which access large files. OTHER RAID LEVELS Many RAID products available today use RAID levels other than the standard ones in their names or descriptions, e.g. RAID 6, RAID 8 etc. Usually these are a combination of the levels described above, or the subsystem that implements one of these may have been enhanced with a feature such as a cache. Of these non-standard levels, RAID 6 and RAID 10 are more widely used than any others. RAID LEVEL 6 RAID 6 is very similar to RAID 5, except that it stores two copies of the parity data rather than one, with each copy being written to a separate drive. This feature significantly improves reliability as three disks in the array must fail for data to be lost; however, the write performance is the lowest of all RAIDs. RAID LEVEL 10 (1/0) Level 10, usually pronounced "one zero", combines the features of RAID level 0 (striping) and RAID level 1 (mirroring) to produce a system with very high performance and high data reliability, but at a relatively high cost.
It should be noted that some manufacturers use the term RAID 6 to define the combination of RAID 0 & 1.
Apricot have introduced this feature into the standard FT//ex 1000, renaming it the FT//ex 1000R. This system employs a very high performance RAID controller from a company called DPT which initially provides a high speed 32 bit PCI local bus adapter with a single channel, 10MB/s SCSI controller and 4Mb of high speed cache. CHANGES TO THE FT//ex 1000 The SCSI cable is replaced by a PCB backplane that allows up to 3 SCSI channels to be fed to the 8 drive bays in the FT//ex. This board provides both a path for the SCSI, the connectors to “hot swap” SCSI devices and switchable terminators that allow a flexible combination of either 1, 2 or 3 channel SCSI. Each of the eight drive trays have extra fittings applied that allow the drive tray to be locked in place and the drive to be powered down to initiate a “hot swap” of a faulty drive. In addition an LED shows when the drive is correctly powered. The on-board Adaptec AIC7870 SCSI controller is employed to control the CD ROM drive and any tape devices fitted in the system while the DPT RAID controller handles the hard disk drives. We would always recommend this configuration as the handling of removable media devices on a RAID controller over-complicates the configuration whilst not providing any advantages over using the on-board controller. Apricot sell systems with 4, 1Gb IBM “Pegasus” drives installed. This is what is considered the optimum configuration for RAID 5 as it allows the use of 3 of the hard disks to be employed as a RAID 5 system and the fourth drive to be as a “hot standby” drive. HOT SWAP Hot Swap is not a facility offered by RAID but is dependant upon the server itself. It requires the ability to power down each hard disk drive independently without shutting down the server. Once the drive is powered down then the drive can be removed without putting too much electrical interference on the, still active, SCSI bus. On the FT//ex 1000R this is achieved by replacement drive trays that include both a power switch for each drive and a PCB edge connector with staggered power/ground/data lines that connect smoothly into the Backplane. HOT STANDBY This is a facility provided by the RAID controller card. If an additional disk is placed on the SCSI bus but is not used for any data, then in the event of one of the other drives failing, the data and parity information is recreated onto the “spare” drive thus allowing for more than one drive to fail without data loss. THE DPT PM3224 SmartRAID CONTROLLER This is a full length, 32 bit PCI 2.0 local bus card containing a 68030 40MHz processor to give full 132MByte/s PCI bus mastering. It supports full SCSI-2 initially on a single channel with the option to upgrade to 2 or even 3 channels. The DPT card will support RAID levels 0, 1 and 5 (not RAID 3). Cache starts with a single 4MB SIMM and is upgradeable to a full 64MB via 72 pin, 36 bit SIMMs with ECC parity. The board has 4 SIMM sockets which will accept either 4MB or 16MB each, allowing a wide range of upgrade options without wasting SIMMs that you have already purchased. The card also includes on-board sensors for temperature and voltage fluctuations and is upgradeable to wide SCSI-2. Shipped with the card are drivers for NetWare 3.x, 4.x, IBM OS/2, SCO UNIX, Microsoft OS/2 1.x, Windows NT, Banyan VINES, UnixWare and many more. With some of these environments it is possible to make changes to the system from a remote site using DPT's Storage Manager software. In the event of a disk failure, it is possible to both log any events as well as broadcast a message to all users or selected groups informing them of any fault in the system, whilst the server continues to run unaffected. Connections can also be made via modem and notifications can be passed via pager or fax.
With RAID hardware and software installed, it is possible to continue normal operation in the event of a disk failure and with Apricot’s FT//ex 1000R it is possible to rectify faults without downing the server. However, in any RAID scenario, whilst the system is running in “degraded” mode (ie. one of the disks has failed and it has not yet updated any hot spare drives that may be available) switching off the wrong drive can cause data loss. It is vital that all hardware engineers who may need to change drives in a RAID system have training on what effects removing the wrong drive can have. Apricot will run a number of seminars explaining RAID further and helping you to recommend just how many drives and in what configuration they are best installed.
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