MAN-005500-000
`Rev. 1.3
`November 21, 1996
`
`CRD-5500
`SCSI RAID Controller
`User's Manual
`
`CMD Technology, Inc.
`1 Vanderbilt
`Irvine, California 92618
`(714) 454-0800
`
`Cisco Systems, Inc. and Quantum Corporation v. Crossroads Systems, Inc.
`CQ-1004 / Page 1 of 92
`
`

`

`Trademarks and Copyright
`CMD, CMD Technology, and CRD-5500 are trademarks of CMD Technology, Inc. All other
`product and company names are trademarks or registered trademarks of other manufacturers.
`
`Copyright © CMD Technology, Inc. 1996. All rights reserved.
`
`CMD reserves the right to make changes to this manual and the equipment described in this
`manual without notice. CMD has made all reasonable efforts to ensure that the information in this
`manual is accurate and complete. CMD will not be liable, however, for any technical or editorial
`errors or omissions made in this manual, or for incidental, special, or consequential damage of
`whatsoever nature, resulting from the furnishing of this manual, or operation and performance of
`equipment in connection with this manual.
`
`FCC Notice
`This equipment has been tested and found to comply with the limits of a Class B digital device,
`pursuant to Part 15 of the FCC rules. These limits are designed to provide reasonable protection
`against harmful interference in a residential installation. This equipment generates, uses and can
`radiate radio frequency energy and, if not installed and used in accordance with the instructions,
`may cause harmful interference to radio communications. However, there is no guarantee that
`interference will not occur in a particular installation. If this equipment does cause harmful
`interference to radio or television reception, which can be determined by turning the equipment off
`and on, the user is encouraged to try to correct the interference by one or more of the following
`measures:
`• Reorient or relocate the receiving antenna.
`• Increase the separation between the equipment and receiver.
`• Connect the equipment into an outlet on a circuit different from that to which the receiver is
`connected.
`• Consult the dealer or an experienced radio/TV technician for help.
`
`Any changes or modifications not expressly approved by the manufacturer could void the user's
`authority to operate the equipment.
`
`Statement of License Limitations
`CMD is licensed under patents by EMC which contain claims directly applicable to controllers.
`The purchase of this controller product does not expressly nor impliedly license the purchaser to
`combine or use the product in combination with any other products or components which
`combination would be covered by EMC patent claims applicable to RAID subsystems. As used
`herein, controllers means a single or multi-processor device or group of functionally interrelated
`devices operable to physically and/or logically configure a plurality of physical mass storage
`devices as one or more independently accessible arrays, and to control the communication of data
`between the array(s) and host(s) or client(s) in a predetermined RAID format or other formats.
`RAID subsystems means a storage subsystem including one or more arrays of physical mass
`storage devices, and one or more controllers associated therewith.
`
`ii
`
`CQ-1004 / Page 2 of 92
`
`

`

`Declaration of Conformity
`CMD Technology, Inc. declares that the equipment described in this document is in conformance
`with the requirements of the European Council Directives listed below:
`
`89/336/EEC
`93/68/EEC
`
`EMC Directive
`EMC Directive
`
`On the approximation of the laws of Member States relating to Electromagnetic Compatibility.
`
`This declaration is based upon compliance of the product to the following standards:
`
`EN 55022, CISPR 22B
`EN 50082-1 IEC 801
`
`RF Emissions Control
`Immunity to Electromagnetic Disturbances
`
`Product Description:
`Model:
`
`RAID SCSI to SCSI Interface
`CRD-5500
`
`Manufacturer:
`
`CMD Technology, Inc.
`1 Vanderbilt
`Irvine, California 92618 (USA)
`
`Warranty
`CMD Technology warrants this product to be free of defects in materials and/or workmanship for a
`period of 3 years from the date of purchase. If the product proves to be defective within the
`warranty period, CMD Technology will either repair or replace it. This warranty covers defects
`incurred in normal use only. Defects, malfunctions, or failures resulting from accidents, misuse, or
`mishandling are not covered. In the event that this product must be repaired or replaced, please
`contact CMD for an RMA number.
`
`Note
`CMD has tested and certified this product in the configuration as delivered. Failure to
`verify any proposed modification or change in configuration with CMD Technical Support
`or Sales may result in improper operation or damage to the product.
`
`iii
`
`CQ-1004 / Page 3 of 92
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`

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`
`iv
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`CQ-1004 / Page 4 of 92
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`

`

`CRD-5500
`
`Table of Contents
`
`1 About the CRD-5500
`
`1-1
`
`1.1 Modular Hardware Design............................................................................................1-1
`
`1.2
`
`1.3
`
`Flexible RAID Set Configuration..................................................................................1-1
`
`Performance Enhancements .......................................................................................1-3
`
`1.3.1 Custom Components .............................................................................................1-3
`
`1.3.2
`
`Efficient Write and Read Algorithms ......................................................................1-3
`
`1.3.2.1 Write-Back Caching.......................................................................................1-4
`
`1.3.2.2 Write Gathering.............................................................................................1-4
`
`1.3.2.3 Write On Top.................................................................................................1-4
`
`1.4 RAID Levels Supported ................................................................................................1-5
`
`1.4.1 RAID 0....................................................................................................................1-5
`
`1.4.2 RAID 1....................................................................................................................1-6
`
`1.4.3 RAID 0+1................................................................................................................1-7
`
`1.4.4 RAID 4....................................................................................................................1-8
`
`1.4.5 RAID 5....................................................................................................................1-9
`
`1.4.6
`
`JBOD....................................................................................................................1-10
`
`1.5 A Few Words About Compatibility ............................................................................1-11
`
`2 Setup Guidelines
`
`2-1
`
`2.1
`
`Installing Modules.........................................................................................................2-1
`
`2.1.1 How To Tell the Modules Apart..............................................................................2-1
`
`2.2 Disk Drives ....................................................................................................................2-3
`
`2.2.1 Drive Geometry ......................................................................................................2-3
`
`2.2.1.1 Drive Geometry Example..............................................................................2-4
`
`2.3
`
`SCSI Bus Setup.............................................................................................................2-4
`
`2.4 Cache .............................................................................................................................2-5
`
`2.4.1 How To Order SIMMs.............................................................................................2-5
`
`2.4.2
`
`Installing SIMMs.....................................................................................................2-6
`
`2.5 Warning Alarm ..............................................................................................................2-6
`
`2.6 Battery Backup..............................................................................................................2-7
`
`Table of Contents
`
`v
`
`CQ-1004 / Page 5 of 92
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`

`

`CRD-5500
`
`2.7
`
`Fan .................................................................................................................................2-8
`
`2.8 Connecting to a Monitor...............................................................................................2-9
`
`2.8.1 Using the Terminal Program in Microsoft Windows ...............................................2-9
`
`3 Configuration Example
`
`3-1
`
`3.1 Connecting the Drives..................................................................................................3-1
`
`3.2
`
`Setting Up the Host Interface.......................................................................................3-1
`
`3.3 Creating a RAID Set ......................................................................................................3-3
`
`3.3.1 How To Create a RAID Set ....................................................................................3-3
`
`3.3.2 Creating a RAID Set with Multiple Partitions..........................................................3-5
`
`3.4 Designating Spare Drives ............................................................................................3-6
`
`3.5 Rebuilding a Disk Drive................................................................................................3-7
`
`4 Using the Monitor Utility
`
`4-1
`
`4.1 Navigating the Menu Tree ............................................................................................4-1
`
`4.2 RAID Information ..........................................................................................................4-1
`
`4.3
`
`Setup Parameters .........................................................................................................4-2
`
`4.3.1 Host Parameters ....................................................................................................4-3
`
`4.3.2
`
`System Parameters................................................................................................4-4
`
`4.3.3 Host LUN Mapping.................................................................................................4-5
`
`4.3.4 Channel Settings....................................................................................................4-5
`
`4.4
`
`System Information ......................................................................................................4-6
`
`4.5 RAID Set Functions ......................................................................................................4-7
`
`4.5.1 Create RAID Set ....................................................................................................4-8
`
`4.5.1.1 Deleting a RAID Set....................................................................................4-11
`
`4.5.2 Rebuild Disk .........................................................................................................4-12
`
`4.5.3
`
`Add Spares ..........................................................................................................4-13
`
`4.6
`
`System Functions .......................................................................................................4-14
`
`4.6.1 Change Password................................................................................................4-14
`
`4.6.2
`
`If You Forget Your Password… ...........................................................................4-14
`
`4.6.3
`
`Load System Code...............................................................................................4-14
`
`4.6.4 Restart System.....................................................................................................4-16
`
`4.6.5
`
`System Shutdown ................................................................................................4-16
`
`4.6.6 Download Drive Firmware....................................................................................4-17
`
`4.7 Disk Utilities ................................................................................................................4-17
`
`vi
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`Table of Contents
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`CQ-1004 / Page 6 of 92
`
`

`

`4.8 Debug Utilities.............................................................................................................4-17
`
`4.9 Rebuild/Create Status.................................................................................................4-17
`
`4.10
`
`Event Log...................................................................................................................4-18
`
`CRD-5500
`
`5 Front Panel
`
`5-1
`
`5.1 Main Menu .....................................................................................................................5-1
`
`5.1.1 RAID Set Status .....................................................................................................5-1
`
`5.1.2
`
`Front Panel Tests...................................................................................................5-1
`
`6 CRD-5500 Redundant RAID Controller Kit
`
`6-1
`
`6.1
`
`Introduction ...................................................................................................................6-1
`
`6.2 Requirements ................................................................................................................6-1
`
`6.3
`
`SIMM Installation...........................................................................................................6-3
`
`6.4 RCC Cable Connection.................................................................................................6-5
`
`6.5
`
`Single-Host Configuration Examples..........................................................................6-7
`
`6.6 Multiple-Host Configuration Examples .....................................................................6-10
`
`6.7 Configuring the Redundant Controller Mode ...........................................................6-16
`
`6.8 Restart After Switchover ............................................................................................6-23
`
`A Warranty
`
`A-1
`
`Return and Repair Policy ...................................................................................................... A-2
`
`Return for Repair ............................................................................................................... A-2
`
`Shipping Charges .............................................................................................................. A-3
`
`Table of Contents
`
`vii
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`CQ-1004 / Page 7 of 92
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`

`

`CRD-5500
`
`This page left blank intentionally.
`
`viii
`
`Table of Contents
`
`CQ-1004 / Page 8 of 92
`
`

`

`1 About the CRD-5500
`
`CRD-5500
`
`The CRD-5500 RAID controller provides high-performance, high-availability access to SCSI disk array
`subsystems along a Fast/Wide SCSI bus. With a modular hardware design and an intuitive configuration
`utility, the controller may be tailored to suit a wide range of storage needs, now and in the future. The CRD-
`5500 is largely compatible with CMD's CRD-5000 RAID controller, so owners of that controller should be
`able to upgrade by doing nothing more than connecting the CRD-5500 to their existing RAID sets.
`
`Note
`The CRD-5500 does not support RAID level 3 or RAID level 0 with a one-block chunk size. Such
`RAID sets created on the CRD-5000 may not be transferred to the CRD-5500.
`
`1.1 Modular Hardware Design
`
`The CRD-5500 employs a modular design for maximum flexibility. You may customize your controller
`using a combination of 8-bit single-ended SCSI I/O modules, 16-bit single-ended modules, and 16-bit
`differential modules. The controller has nine I/O module slots. One of these slots is reserved for use as a host
`channel, and five are reserved for use as drive channels. The other three can serve as host or drive channels,
`depending on how they are configured in the controller's onboard configuration utility. This means that you
`can have one host channel and as many as eight disk channels. Or you can trade as many as three of the drive
`channels for host channels, resulting in a possible configuration of four host channels and as many as five
`drive channels.
`
`1.2 Flexible RAID Set Configuration
`
`In addition to its flexible hardware design, the CRD-5500's firmware offers the user the flexibility to
`configure RAID sets in many different ways:
`• RAID sets may comprise drives from any drive channel and SCSI ID.
`• A RAID set may contain all the drives connected to the controller, a single drive, or any number of drives
`in between.
`• The controller supports RAID Levels 0, 1, 0+1, 4, and 5. It also supports “Just a Bunch of Drives”
`(JBOD), which permits you to connect standalone disk drives (such as a system disk) to the CRD-5500
`without making them members of a RAID set.
`• Each RAID set may be partitioned into smaller redundancy groups.
`• The controller's Host LUN Mapping feature makes it possible to map RAID sets differently to each host.
`You make the same redundancy group show up on different LUNs to different hosts, or make a
`redundancy group visible to one host but not to another.
`• Any drive may be designated as a hot or warm spare. Spares are global, meaning that in the event of a
`drive failure, the CRD-5500 will search for the first suitable spare on any channel or SCSI ID and
`automatically begin rebuilding the failed drive's data.
`
`Figure 1-1 shows an example of how the controller's drives may be combined in various ways to suit
`different needs.
`
`About the CRD-5500
`
`1-1
`
`CQ-1004 / Page 9 of 92
`
`

`

`CRD-5500
`
`Host
`
`CRD-5500
`
`20
`
`30
`
`40
`
`50
`
`6 0
`
`70
`
`80
`
`21
`
`3 1
`
`41
`
`51
`
`61
`
`7 1
`
`81
`
`22
`
`3 2
`
`42
`
`52
`
`62
`
`7 2
`
`82
`
`23
`
`33
`
`43
`
`53
`
`6 3
`
`73
`
`83
`
`24
`
`3 4
`
`44
`
`54
`
`64
`
`7 4
`
`84
`
`25
`
`3 5
`
`45
`
`55
`
`65
`
`7 5
`
`85
`
`26
`
`3 6
`
`46
`
`56
`
`66
`
`7 6
`
`86
`
`20
`
`7 5
`
`82
`
`53
`
`42
`
`81
`
`R AID 4 Set
`
`85
`
`8 0
`
`66
`
`6 0
`
`21
`
`24
`
`45
`
`3 1
`
`51
`
`3 4
`
`RAID 5 Set
`(4 Partitions)
`
`62
`
`33
`
`RAID 1 M irror Pair
`
`44
`
`72
`
`43
`
`JBOD
`Drives
`73
`36
`
`23
`
`52
`
`32
`
`61
`
`40
`
`70
`
`RAID 0+1
`Set
`
`42
`
`71
`G lobal
`Spares
`30
`
`22
`
`25
`
`8 3
`
`35
`
`RAID 0 Set
`
`Logical Arrangement of Disk Drives
`
`Physical Arrangement of Disk Drives
`
`Figure 1-1: How the controller views RAID sets
`
`Figure 1-2 shows how you can connect as many as four hosts to the CRD-5500. By using the controller's
`Host LUN Mapping feature, you can assign redundancy groups to a particular host. This makes it possible
`for hosts running incompatible operating systems to use the same CRD-5500 controller. If the hosts are part
`of a VMS VAXCluster™ they can share access to all of the redundancy groups.
`
`1-2
`
`About the CRD-5500
`
`CQ-1004 / Page 10 of 92
`
`

`

`Host
`
`Host
`
`Host
`
`Host
`
`CRD-5500
`
`0 1 2
`
`3
`
`CRD-5500
`
`Figure 1-2: A multi-hosting example
`
`1.3 Performance Enhancements
`
`The CRD-5500 employs a number of techniques to achieve as much performance as possible from its design.
`
`1.3.1 Custom Components
`
`To increase performance and reliability, the CRD-5500's core functions have been encapsulated in four
`custom ASIC (Application Specific Integrated Circuits) components.
`
`XOR ASIC: Used in the Exclusive-Or parity calculations employed by RAID levels 4 and 5.
`
`DMA ASIC: Controls the data path hardware for the various I/O ports.
`
`CPU Interface ASIC:
`
`Supports the controller's MIPS R3000 RISC central processing unit.
`
`Memory Controller ASIC: Controls the memory system and supports data movement on the internal
`bus at a maximum burst rate of 80 MB/second and a maximum sustainable rate of 60 MB/second.
`
`1.3.2 Efficient Write and Read Algorithms
`
`Standard RAID write operations that involve parity, such as those in RAID levels 4 and 5, require multiple,
`time-consuming steps:
`
`About the CRD-5500
`
`1-3
`
`CQ-1004 / Page 11 of 92
`
`

`

`CRD-5500
`
`1 Read data from the parity drive.
`
`2 Read existing data from the target data drives.
`
`3 Exclusive-Or the old parity, old data, and new data to generate new parity data.
`
`4 Write the new parity data to the parity drive.
`
`5 Write the new data to the target data drives.
`
`The CRD-5500 uses several techniques to streamline write operations and significantly improve
`performance. All the techniques use the controller's onboard cache, which can contain up to 512 megabytes
`of memory in the form of standard 72-pin, 60-nanosecond SIMMs.
`
`Note
`The CRD-5500 will not operate without at least one 4MB SIMM installed in its cache. Nor will it
`operate without either a battery backup or an uninterruptible power supply connected to the
`controller. Without a backup, data stored in the cache but not yet written to the disk drives would
`be lost in the event of a power interruption.
`
`1.3.2.1 Write-Back Caching
`When the host sends data to be written to a redundancy group, the controller stores the data in its cache and
`immediately reports to the host it has completed the write. The controller eventually writes the data to the
`disk drives, when the write can be done most efficiently or when the controller must flush the cache to make
`room for other data or to prepare for a shutdown.
`
`Write-back caching makes the host more responsive to the user, since the host does not have to wait for a
`lengthy RAID write before processing another task.
`
`1.3.2.2 Write Gathering
`The CRD-5500 will attempt to consolidate multiple writes destined for contiguous blocks and then write the
`entire data block in one operation. The controller stores the data in cache until it performs the write. Ideally,
`the controller will wait until it has gathered enough data to fill an entire stripe. This enables the controller to
`avoid reading from the parity and data drives before making the write. All it has to do is calculate parity from
`the data it already has in its cache, then write the data and parity to the drives. Even if the controller cannot
`accumulate enough data to fill a stripe, the consolidation of multiple small writes can reduce the number of
`read/write operations that must take place.
`
`1.3.2.3 Write On Top
`If the host commands that data be written to disk, and data for that address is pending in the controller's
`cache, the controller writes the new data on top of the old in the cache. Only the new data is eventually
`written to the disk drives.
`
`1-4
`
`About the CRD-5500
`
`CQ-1004 / Page 12 of 92
`
`

`

`CRD-5500
`
`1.4 RAID Levels Supported
`
`The CRD-5500 supports the following RAID levels:
`
`RAID Level
`
`Description
`
`0
`
`1
`
`0+1
`
`4
`
`5
`
`Striping without parity
`
`Mirroring
`
`Striping and mirroring
`
`Striping with fixed parity drive
`
`Striping with floating parity drive
`
`JBOD
`
`“Just a Bunch of Drives”
`
`Note
`The CRD-5500 stripes data in multi-block chunk sizes. If you are upgrading from a CRD-5000 and
`have a Level 0 RAID set with a single-block chunk size, you will have to recreate the RAID set to
`connect it to the CRD-5500.
`
`1.4.1 RAID 0
`
`RAID 0 breaks up data into smaller chunks and then writes each chunk to a different drive in the array. The
`size of each chunk is determined by the controller's chunk size parameter, which you set in the course of
`creating a RAID set.
`
`The advantage of RAID 0 is its high bandwidth. By breaking up a large block of data into smaller chunks,
`the controller can use multiple drive channels to write the chunks to the disk drives. Furthermore, RAID 0
`involves no parity calculations to complicate the write operation. Likewise, a RAID 0 read operation
`employs multiple drives to assemble a single, large data block. This makes RAID 0 ideal for applications,
`such as graphics, video, and imaging, that involve the writing and reading of huge, sequential data blocks.
`
`About the CRD-5500
`
`1-5
`
`CQ-1004 / Page 13 of 92
`
`

`

`CRD-5500
`
`R A ID 0 W rite
`
`C ontroller Divides Data into
`Chunksized Units
`
`H ost
`D ata
`
`Striped Data
`W ritten to Array
`
`There's Still D ata Left S o
`Controller Repeats
`The P rocess
`
`Striped Data
`W ritten to Array
`
`Figure 1-3: Diagram of a RAID 0 write
`
`Warning
`The lack of parity means that a RAID 0 disk array offers absolutely no redundancy and thus cannot
`recover from a drive failure.
`
`1.4.2 RAID 1
`
`RAID 1 (also known as mirroring or shadowing) takes data sent by the host and duplicates it on all the drives
`in the array. This results in a high degree of data availability, since you can lose all but one drive in the array
`and still have full access to your data. This comes at a price: a RAID 1 array requires multiple drives to
`achieve the storage capacity of a single drive.
`
`1-6
`
`About the CRD-5500
`
`CQ-1004 / Page 14 of 92
`
`

`

`R A ID 1 W rite
`
`CRD-5500
`
`Host
`Data
`
`Controller W rites Data
`from Cache to
`All Drives in the Array
`
`Figure 1-4: Diagram of a RAID 1 write
`
`A RAID 1 array will show up on the monitor as “degraded” when at least one drive fails, even if two or more
`members of the redundancy group remain in good working order. As long as at least two working drives
`remain in the array, you may continue to run the array in degraded mode without putting your data in
`jeopardy.
`
`1.4.3 RAID 0+1
`
`As its name implies, RAID 0+1 combines RAID 0 (striping) with RAID 1 (mirroring). In a RAID 0+1 write,
`the controller breaks up the data block from the host into smaller chunks, then writes the chunks to half the
`drives in the array, while writing duplicate chunks to the remaining drives.
`
`R AID 0+1 W rite
`
`Controller Divides Data into
`Chunksized Units
`
`Host
`D ata
`
`Striped Data Written
`to Half the Drives
`
`S triped D ata M irrored
`to R em aining D rives
`
`Figure 1-5: Diagram of a RAID 0+1 write
`
`In the event of a drive failure, a RAID 0+1 array will enter degraded mode and continue to operate by
`substituting the failed drive with its mirror.
`
`When the controller creates a RAID 0+1 set, it first sorts the drives by channel number and SCSI ID. Then it
`stripes the data across every other drive and forms a mirrored pair with the first two drives, another mirrored
`
`About the CRD-5500
`
`1-7
`
`CQ-1004 / Page 15 of 92
`
`

`

`CRD-5500
`
`pair with the second two drives, and so on. Table 1-1 describes how the controller uses the drives in a RAID
`0+1 set.
`
`Table 1-1: RAID 0+1 Example
`
`Drives Selected
`
`Function
`
`Channel 1, ID 0
`
`Channel 1, ID 1
`
`Channel 1, ID 2
`
`Channel 2, ID 0
`
`Channel 2, ID 1
`
`Channel 2, ID 2
`
`First member of stripe set
`
`Mirror of Channel 1, ID 0
`
`Second member of stripe set
`
`Mirror of Channel 1, ID 2
`
`Third member of stripe set
`
`Mirror of Channel 2, ID 1
`
`1.4.4 RAID 4
`
`RAID 4 breaks up host data into chunks, calculates parity by performing an exclusive-or on the chunks, and
`then writes the chunks to all but one drive in the array and the parity data to the last drive. When the host
`request data from the disk drives, the controller retrieves the chunks containing the addressed data,
`reconstitutes the data from the chunks, and passes the data to the host.
`
`R A ID 4 W rite
`
`H ost
`D ata
`
`C ontroller Divides Data into
`Chunksized Units
`and Calculates Parity
`
`XO r
`Parity =
`
`P
`
`Striped Data
`and Parity
`W ritten to Array
`
`P
`
`There's Still D ata Left S o
`Controller Repeats
`The P rocess
`
`XO r
`Parity =
`
`P
`
`Striped Data
`and Parity
`W ritten to Array
`
`P
`
`P
`
`Figure 1-6: Diagram of a RAID 4 write
`
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`About the CRD-5500
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`CQ-1004 / Page 16 of 92
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`CRD-5500
`
`In the event of a single drive failure, a RAID 4 array will continue to operate in degraded mode. If the failed
`drive is a data drive, writes will continue as normal, except no data will be written to the failed drive. Reads
`will reconstruct the data on the failed drive by performing an exclusive-or operation on the remaining data in
`the stripe and the parity for that stripe. If the failed drive is a parity drive, writes will occur as normal, except
`no parity will be written. Reads will simply retrieve data from the data disks. There will be no deterioration
`in controller performance while a RAID set is in degraded mode.
`
`In general, RAID 4 is best suited for applications such as graphics, imaging, or video that call for reading
`and writing huge, sequential blocks of data. However, you may find that RAID 4 is preferable to RAID 5
`even for applications characterized by many small I/O operations, such as transaction processing. This is due
`to the CRD-5500's intelligent caching, which efficiently handles small I/O reads and writes, and to the
`relatively less complex algorithms needed to implement RAID 4.
`
`The benefits of RAID 4 disappear when you have many, small I/O operations scattered randomly and widely
`across the disks in the array. RAID 4's fixed parity disk becomes a bottleneck in such applications, as the
`following example illustrates. Let's say the host instructs the controller to make two small writes. The writes
`are widely scattered, involving two different stripes and different disk drives. Ideally, you would like both
`writes to take place at the same time, but RAID 4 makes this impossible, since the writes must take turns
`accessing the fixed parity drive. For this reason, RAID 5 is the better choice for widely scattered, small write
`operations.
`
`Warning
`RAID 4 can withstand a single drive failure and handle I/O activity without interruption in degraded
`mode until the failed drive is rebuilt. If a second drive fails while the RAID set is in degraded mode,
`the entire RAID set will fail.
`
`1.4.5 RAID 5
`
`RAID 5 addresses the bottleneck issue for barrages of widely scattered, small I/O operations. Like RAID 4,
`RAID 5 breaks data up into chunks, calculates parity, and then writes the chunks in stripes to the disk drives,
`saving one drive on each stripe for the parity data. Unlike RAID 4, however, RAID 5 changes the parity
`drive on each stripe. This means, for instance, that a write operation involving drive 2 on stripe 1 can
`conceivably take place at the same time as a write involving drive 3 on stripe 2, since they would be
`addressing different parity drives.
`
`About the CRD-5500
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`1-9
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`CQ-1004 / Page 17 of 92
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`

`CRD-5500
`
`R A ID 5 W rite
`
`H ost
`D ata
`
`C ontroller Divides Data into
`Chunksized Units
`and Calculates Parity
`
`XO r
`Parity =
`
`P
`
`Striped Data
`and Parity
`W ritten to Array
`
`P
`
`There's Still D ata Left S o
`Controller Repeats
`The P rocess
`
`XO r
`Parity =
`
`P
`
`Striped Data
`and Parity
`W ritten to Array
`
`P
`
`P
`
`Figure 1-7: Diagram of a RAID 5 write
`
`RAID 5 handles drive failures in the same manner as RAID 4, except the parity drive is different for each
`stripe. The controller either uses the parity information on a stripe to reconstruct its data or simply reads the
`data as normal, depending on location of the stripe's parity drive.
`
`While RAID 5 is ideally suited for applications with many, small I/O operations, there is no reason why it
`cannot function equally well for applications with large, sequential I/O operations. This makes RAID 5 an
`excellent all-purpose RAID level.
`
`Warning
`RAID 5 can withstand a single drive failure and handle I/O activity without interruption in degraded
`mode until the failed drive is rebuilt. If a second drive fails while the RAID set is in degraded mode,
`the entire RAID set will fail.
`
`1.4.6 JBOD
`
`JBOD, which stands for “Just a Bunch of Disks,” makes it possible to connect one or more standalone disk
`drives to the controller. A JBOD disk drive is not part of a redundancy group, even though the controller
`assigns a redundancy group number to the drive. This number becomes that logical unit number (LUN) that
`the host will use to address the drive. (You may map any redundancy group number to another LUN using
`the Host LUN Mapping feature of the monitor utility.)
`
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`About the CRD-5500
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`One use for JBOD is to connect a system disk drive to the CRD-5500. The drive does not become part of a
`RAID set, but it is made available to the host on the same SCSI bus as the other devices controlled by the
`CRD-5500.
`
`CRD-5500
`
`1.5 A Few Words About Compatibility
`
`The CRD-5500 is designed to be compatible with the CRD-5000, an earlier RAID controller from CMD
`Technology. In most cases, CRD-5000 users should be able to run their existing RAID sets on the
`CRD-5500 without recreating them. A handful of restrictions apply to this backwards compatibility:
`• The CRD-5500 does not support RAID 3 or RAID 0 with single-block chunk sizes. RAID sets created on
`the CRD-5000 with either of these configurations will not operate on the CRD-5500.
`• Disk drives connected to the CRD-5500 must support “read long” and “write long.” Most SCSI disk
`drives meet this criterion, but you should check the specifications for your older drives.
`• The CRD-5500 uses high-density connectors on all its I/O modules. You may need to replace some or all
`of the cables from your CRD-5000 system.
`• You will not be able to use the memory modules from your old controller. The CRD-5500 requires 72-
`pin, 36-bit, 60-nanosecond SIMMs. The CRD-5000 uses 30-pin,