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PERC 5 Features

Dell™ PowerEdge™ Expandable RAID Controller 5/i and 5/E User's Guide

  PERC 5 Controller Features

  Battery Management

  RAID Configuration Information

  Fault Tolerance Features

  Patrol Read


This section describes the features of the The Dell™ PowerEdge™ Expandable RAID Controller (PERC) 5 family of controllers, such as the configuration options, disk array performance, hardware specifications, redundant array of independent disks (RAID) management utilities, and operating system software drivers.


PERC 5 Controller Features

This section describes the hardware configuration features for the PERC 5 controllers.

Table 2-1 compares the configurations for the controllers.

Table 2-1. PERC 5 Controller Comparisons 

Specification

PERC 5/E Adapter

PERC 5/i Adapter

PERC 5/i Integrated

RAID Levels

0, 1, 5, 10, 50

0, 1, 5, 10, 50

0, 1, 5, 10, 50

Enclosures per Port

Up to 3 enclosures

N/A

N/A

Ports

2 x4 external wide port

2 x4 internal wide port

2 x4 internal wide port

Processor

Intel® IOP333 I/O processor with Intel XScale Technology

Intel IOP333 I/O processor with Intel XScale Technology

Intel IOP333 I/O processor with Intel XScale Technology

Battery Backup Unit

Yes, Transportable

Yesa

Yes

Cache Memory

256 MB DDR2 cache memory size

256 MB DDR2 cache memory size

256 MB DDR2 cache memory size

Cache Function

Write-back, write-through, adaptive read ahead, non-read ahead, read ahead

Write-back, write-through, adaptive read ahead, non-read ahead, read ahead

Write-back, write-through, adaptive read ahead, non-read ahead, read ahead

Maximum Number of Drives per Array

Up to 32 drives per array

Up to 32 drives per array

Up to 32 drives per array

Maximum Number of Arrays per Disk Group

Up to 8 arrays (in a spanned configuration)

Up to 8 arrays (in a spanned configuration)

Up to 8 arrays (in a spanned configuration)

Maximum Number of Virtual Disks per Disk Group

Up to 16 virtual disks per disk group

RAID 0=16

RAID 1=16

RAID 5=16

RAID 10= 1

RAID 50=1

Up to 16 virtual disks per disk group

RAID 0=16

RAID 1=16

RAID 5=16

RAID 10=1

RAID 50=1

Up to 16 virtual disks per disk group

RAID 0=16

RAID 1=16

RAID 5=16

RAID 10=RAID 50=1

Multiple Virtual Disks per Controller

Up to 64 virtual disks per controller

Up to 64 virtual disks per controller

Up to 64 virtual disks per controller

Support for x8 PCI Express host interface

Yes

Yes

Yes

Online Capacity Expansion

Yes

Yes

Yes

Dedicated and Global Hot Spares

Yes

Yes

Yes

Hot Swap Devices Supported

Yes

Yes

Yes

Non-Disk Devices Supported

No

No

No

Mixed Capacity Physical Disks Supported

Yes

Yes

Yes

Hardware Exclusive-OR (XOR) Assistance

Yes

Yes

Yes

a The PERC 5/i Adapter supports a battery backup unit (BBU) on selected systems only. See the documentation that shipped with the system for additional information.

NOTE: The maximum length of cable that you can use for Serial Attached SCSI (SAS) is 4 meters (13 feet) from port to port. This applies only to external cables.
NOTE: The maximum array size is limited by the maximum number of drives per array (32) and the maximum number of spans per disk group (8), along with the size of the physical drives. This limits the number of spans in RAID 10 to eight, giving a total of 16 drives per virtual disk.

Compatibility With Virtual Disks Created on Existing PERC 5 Controllers

The PERC 5 controllers recognize and use virtual disks created on existing PERC 5 controllers without risking data loss, corruption, redundancy, or configuration loss. Similarly, the virtual disks created on the controllers can be transferred to other PERC 5 controllers.

NOTE: For more information about compatibility, contact your Dell Technical Support Representative.

SMART Technology

The Self-Monitoring Analysis and Reporting Technology (SMART) feature monitors the internal performance of all motors, heads, and physical disk electronics to detect predictable physical disk failures. This feature helps monitor physical disk performance and reliability, and protects the data on the physical disk. When problems are detected on a physical disk, you can replace or repair the physical disk without losing any data.

SMART-compliant physical disks have attributes for which data (values) can be monitored to identify changes in values and determine whether the values are within threshold limits. Many mechanical and electrical failures display some degradation in performance before failure.

There are numerous factors that relate to predictable physical disk failures, such as a bearing failure, a broken read/write head, and changes in spin-up rate. In addition, there are factors related to read/write surface failure, such as seek error rate and excessive bad sectors.

NOTE: See www.t10.org for detailed information about Small Computer System Interface (SCSI) interface specifications and www.t13.org for Serial Attached ATA (SATA) interface specifications.

Background Initialization

Background Initialization (BGI) is a process to correct parity on the virtual disks. BGI is an automated check for media errors in which parity is created and written in the process. BGI does not run on RAID 0 virtual disks.

NOTE: You cannot permanently disable BGI. If you cancel BGI, it automatically restarts within five minutes. See Stopping Background Initialization for information on stopping BGI.

The background initialization rate is controlled by the storage management software. You must stop an ongoing background initialization before you change the rate, or the rate change will not take effect. After you stop background initialization and change the rate, the rate change will take effect when the background initialization restarts automatically.

NOTE: Unlike initialization of virtual disks, background initialization does not clear data from the physical disks.
NOTE: Consistency Check and Background Initialization perform the same function. The difference between them is that Background Initialization cannot be started manually, while Consistency Check can.

LED Operation

The LED on the physical disk carrier indicates the state of each physical disk. For internal storage, see your system documentation for more information about the blink patterns.

For blink patterns on the Dell PowerVault™ MD1000, see the Dell PowerVault MD1000 Hardware Owner's Manual. For blink patterns on the Dell PowerVault MD3000, see the Dell PowerVault MD3000 Hardware Owner's Manual.

Disk Roaming

The PERC 5 controllers support moving physical disks from one cable connection or backplane slot to another on the same controller. The controllers automatically recognize the relocated physical disks and logically place them in the proper virtual disks that are part of the disk group. You can perform disk roaming only when the system is shut down.

Perform the following steps to use disk roaming.

  1. For proper shutdown, turn off the power to the system, physical disks, enclosures, and system components, and then disconnect the power cords from the system.

  2. Move the physical disks to different positions on the backplane or enclosure.

  3. Perform a safety check. Make sure the physical disks are inserted properly and perform correctly.

  4. Turn on the system.

The controller detects the RAID configuration from the configuration data on the physical disks.

Disk Migration

The PERC 5 controllers support migration of virtual disks from one controller to another without taking the target controller offline. However, the source controller must be offline prior to performing the disk migration. The controller can import a virtual disk that is in optimal or degraded state. A virtual disk in offline state cannot be imported.

NOTE: The PERC 5 controllers are not backward compatible with previous SCSI PERC RAID controllers.

When a controller detects a physical disk with a pre-existing configuration, it flags the physical disk as foreign and generates an alert indicating that a foreign disk was detected.

Dedicated hot spares are imported as global hot spares. The firmware generates an alert to indicate a change in hot spare configuration.

Perform the following steps to use disk migration.

  1. Turn off the system that contains the source controller.

  2. Move the appropriate physical disks from the source controller to the target controller.

The system with the target controller can be running while inserting the physical disks.

  1. The storage management application will flag the inserted disks as foreign disks.

  2. Use the storage management application to import the detected foreign configuration.

NOTE: Ensure the complete set of physical disks that form the virtual disk are migrated.

Alarm Alert in Case of Physical Disk Failures

An audible alarm is available on the PERC 5/E Adapter to alert you of key critical and warning events involving the virtual disk or physical disk problems. You can use the Basic Input/Output System (BIOS) Configuration Utility and management application to enable, disable, or silence the on-board alarm tone.

NOTE: See Audible Alarm Warnings for information about audible alarm codes.

Battery Management

The Transportable Battery Backup Unit (TBBU) is a cache memory module with an integrated battery pack that enables you to transport the cache module with the battery into a new controller. The TBBU protects the integrity of the cached data on the PERC 5/E Adapter by providing backup power during a power outage.

The Battery Backup Unit (BBU) is a battery pack that protects the integrity of the cached data on the PERC 5/i Adapter and PERC 5/i Integrated controllers by providing backup power during a power outage. Unlike the TBBU, the BBU is not directly attached to the cache memory module and is therefore not transportable with the controller.

The TBBU and BBU offer an inexpensive way to protect the data on the memory module. The lithium battery provides a way to store more power in a smaller form factor than previous batteries.

See Transferring a TBBU Between Controllers for detailed procedures about handling controller cache in case of a controller failure.

Introduction to Write Cache Policy

The cache controller writes a block of data to cache memory, which is much faster than writing to the physical disk. The cache controller sends an acknowledgement of data transfer completion to the host system.

Write-Back versus Write-Through

In write-through caching, the controller sends a data transfer completion signal to the host system when the disk subsystem has received all the data in a transaction. The controller then writes the cached data to the storage device when system activity is low or when the write buffer approaches capacity.

In write-back caching, the controller sends a data transfer completion signal to the host when the controller cache has received all the data in a transaction. The cached data is not written to the storage device.

The risk of using write-back cache is that the cached data can be lost if there is a power failure before it is written to the storage device. This risk is eliminated by using a battery backup unit on selected PERC 5 controllers. Refer to Table 2-1 for information on which controllers support a battery backup unit.

Write-back caching has a performance advantage over write-through caching.

NOTE: The default cache setting is write-back caching.
NOTE: Certain data patterns and configurations perform better in a write-through cache policy.

How Firmware Manages Cache

Firmware manages cache based on the condition of the battery. Learn Cycle is a battery calibration operation performed by the controller periodically (approximately every 3 months) to determine the condition of the battery.

Conditions Under Which Write-Back is Employed

Write-back caching is used under all conditions in which the battery is present and in good condition.

Conditions Under Which Write-Through is Employed

Write-through caching is used under all conditions in which the battery is missing or in a low-charge state. Low-charge state is when the battery is not capable of maintaining data for at least 24 hours in the case of a power loss.

Conditions Under Which Forced Write-Back With No Battery is Employed

Write-Back mode is available when the user selects Force WB with no battery. When Forced Write-Back mode is selected, the virtual disk is in Write-Back mode even if the battery is present and in good condition (healthy), or if a learn cycle is in process. Dell recommends that you use a power backup system when forcing Write-Back to ensure that there is no loss of data if the system suddenly loses power.

Learn Cycle Completion Time Frame

The time frame for completion of a learn cycle is a function of the battery charge capacity and the discharge/charge currents used. For PERC 5, the expected time frame for completion of a learn cycle is approximately seven hours and consists of the following parts:

Learn cycles shorten as the battery capacity derates over time.

NOTE: See the storage management application for additional information.

During the discharge phase of a learn cycle, the PERC 5 battery charger is disabled. In this phase, the battery voltage is monitored through the smart battery bus (SMBus) using the battery gas-gauge. When the battery reaches the discharge capacity threshold (DCT), the dummy load is disabled and the battery charger is re-enabled. At this point, the battery charger detects that the battery is below the fast charge trigger voltage (FCTV) and initiates a fast-charge of the battery. The learn cycle completes once the battery fast charge is completed.


RAID Configuration Information

Table 2-2 lists the configuration features for the PERC 5 controllers.

Table 2-2. Features for RAID Configuration 

Specification

PERC 5/E Adapter

PERC 5/i Adapter and 5/i Integrated

Number of virtual disks supported

Up to 64 virtual disks per controller

Up to 64 virtual disks per controller

NOTE: The number of physical disks on a controller is limited by the backplane on which the card is attached.

Online RAID level migration

Yes

Yes

Disk roaming

Yes

Yes

No reboot necessary after capacity expansion

Yes

Yes

User-specified rebuild rate

Yes

Yes


Fault Tolerance Features

Table 2-3 lists the features that provide fault tolerance to prevent data loss in case of a failed physical disk.

Table 2-3. Fault Tolerance Features 

Specification

Feature

Support for SMART

Yes

Support for Patrol Read

Yes

Physical disk failure detection

Automatic

Physical disk rebuild using hot spares

Automatic

Parity generation and checking (RAID 5 only)

Yes

Battery backup for controller cache to protect configuration data

Yesa

Hot-swap manual replacement of a physical disk unit without bringing the system down

Yes

a The PERC 5/i Adapter supports a battery backup unit (BBU) on selected systems only. For additional information,
see documentation that was shipped with the system.

Physical Disk Hot Swapping

Hot swapping is the manual substitution of a replacement unit in a disk subsystem for a defective one, where the substitution can be performed while the subsystem is running (performing its normal functions).

NOTE: The backplane or enclosure must support hot swapping in order for the PERC 5 controllers to support hot swapping.
NOTE: Ensure that SAS drives are replaced with SAS drives and SATA drives are replaced with SATA drives.
NOTE: While swapping a disk, ensure that the new disk is of equal or greater capacity than the disk that is being replaced.

Failed Physical Disk Detection

The firmware automatically detects and rebuilds failed physical disks. Automatic rebuilds can be performed transparently with hot spares. If you have configured hot spares, the controllers automatically try to use them to rebuild failed physical disks.


Patrol Read

The Patrol Read function is designed as a preventive measure that includes review of your system for possible physical disk errors that could lead to physical disk failure and damage data integrity. The Patrol Read operation can find and possibly resolve any potential problem with physical disks prior to host access. This can enhance overall system performance because error recovery during a normal I/O operation may not be necessary. You can use the storage management application to perform Patrol Read functions.

Patrol Read Behavior

The following is an overview of Patrol Read behavior:

  1. Patrol Read runs on all disks on the controller that are configured as part of a virtual disk including hot spares. Patrol Read does not run on unconfigured physical disks. Unconfigured disks are those that are not part of a virtual disk or are in a ready state.

  2. Patrol Read adjusts the amount of controller resources dedicated to Patrol Read operations based on outstanding disk I/O. For example, if the system is busy processing I/O operation, then Patrol Read will use fewer resources to allow the I/O to take a higher priority.

  3. Patrol Read operates on all configured physical disks on the controller and there is no method to deselect disks.

  4. In a Patrol Read iteration, Patrol Read will restart from zero percent if in Auto mode. In Manual mode, Patrol Read does not restart on a reboot. Use Manual mode if you have selected a window of time dedicated to running Patrol Read.

Configuration

You can use the storage management application to select the Patrol Read options. Use Patrol Read options to set automatic or manual operation, or disable Patrol Read. The following sections describe Patrol Read functions and operations available in the Storage Management application.

NOTE: See the storage management application's documentation for more information about the Patrol Read configuration features available.

Patrol Read Modes

The following describes the scheduling details for Patrol Read:

  1. By default the controller sets the Patrol Read to Auto mode. You can set the Patrol Read to either Auto or Manual mode.

  2. In Auto mode, Patrol Read runs continuously on the system and is scheduled to start a new Patrol Read within seven days after the last iteration is completed.

  3. When Patrol Read Mode is changed from Auto to Manual, or Auto to Disabled, the Next execution will start at: field will be set to N/A.

Behavior Details

The behavior details of Patrol Read are as follows:

  1. Setting Patrol Read in Manual mode does not start Patrol Read. It only sets the mode so that you can select Start whenever you want to run Patrol Read. When the mode is set to Manual, it remains in that mode until you change it.

  2. Setting the mode to Automatic starts Patrol Read. When the Patrol Read operation is complete, it will set itself to run within seven days of the last iteration.

Blocked Operations

If any of the following conditions exist, then Patrol Read will not run on any of the affected disks:


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