Drive Sparing in EMC Symmetrix DMX-3 and DMX-4 Systems

Transcription

1 Applied Technology Abstract Drive sparing significantly increases data protection and availability. EMC Symmetrix systems support dynamic and permanent sparing. This white paper explains the benefits of both sparing methods, and describes the sparing processes in Symmetrix DMX-3 and DMX-4 systems running Enginuity 5772 and later. Configuration rules and considerations are also discussed. June 2009

2 Copyright 2008, 2009 EMC Corporation. All rights reserved. EMC believes the information in this publication is accurate as of its publication date. The information is subject to change without notice. THE INFORMATION IN THIS PUBLICATION IS PROVIDED AS IS. EMC CORPORATION MAKES NO REPRESENTATIONS OR WARRANTIES OF ANY KIND WITH RESPECT TO THE INFORMATION IN THIS PUBLICATION, AND SPECIFICALLY DISCLAIMS IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Use, copying, and distribution of any EMC software described in this publication requires an applicable software license. For the most up-to-date listing of EMC product names, see EMC Corporation Trademarks on EMC.com All other trademarks used herein are the property of their respective owners. Part Number H Applied Technology 2

3 Table of Contents Executive summary...4 Introduction...4 Audience... 5 Terminology... 5 Dynamic sparing...6 Process... 6 Benefits... 7 Considerations... 8 Configuration file settings... 8 Permanent sparing...8 Process... 8 Benefits Replenishment Process for Permanent Spares EMC Certified Data Erasure for Symmetrix Disks Considerations Configuring spares...12 Permanent spare coverage Configuration file drive map Query Spare Pool Status script Spare rules DMX-4 spare rules DMX-3 spare rules Flash drives and sparing Conclusion...14 Applied Technology 3

4 Executive summary As the demands from internal and external customers force IT organizations to deliver the very highest service levels, availability expectations have increased to where any interruption to the access of information is unacceptable. Binding service-level agreements (SLAs) commit IT organizations to deliver agreed-to measurable support metrics. Even in the face of a component failure, these organizations are required to provide continuous data availability without compromise. IT executives around the globe have recognized that downtime is not only measured in minutes or hours, but is also calculated as potential revenue loss, missed opportunities, or dissatisfied customers. Any operational impacts resulting from a component failure must be completely transparent to the individual applications and users relying on information availability to drive the business. Symmetrix systems implement drive sparing to increase data availability without compromising performance or functionality. Sparing protects against multiple drive failure in a RAID group before a failed or failing drive can be replaced and completely resynchronized. Symmetrix DMX-3 and DMX-4 systems offer two types of sparing: dynamic sparing and permanent sparing. Dynamic sparing temporarily provides an additional copy of the contents of a failing drive. Dynamic sparing allows the data to remain in a protected or N+1 state while a failing drive is replaced. Permanent sparing is a process that replaces a failing drive with a spare drive. Permanent sparing is initiated automatically upon detection of certain error conditions, reducing the amount of time that a failed or failing drive remains in the system. Enginuity will determine which sparing method to employ, or may use a combination of both methods, in order to provide the best protection possible. Introduction Symmetrix has many built-in design features to provide protection against data unavailability due to single component or power failure. A redundant design allows the Symmetrix to remain online and operational during service events and upgrades. Proactive error detection and remote support allow components to be replaced before actual failures occur. Symmetrix Enginuity, the operating environment for Symmetrix systems, offers a variety of additional features dedicated to data protection and availability. RAID 1, RAID 5, and RAID 6 configurations provide local data redundancy. The EMC TimeFinder and SRDF families of software are the most powerful suites of local and remote storage replication solutions available in the industry, providing the ability to create point-in-time local copies, as well as remotely replicated copies to guard against primary site disasters and outages. Symmetrix systems also have the capability to reserve drives as standby spares. These drives are not useraddressable and are configured in various physical locations for maximum effectiveness. Sparing significantly increases data availability without affecting performance. Symmetrix DMX-3 and DMX-4 systems support both dynamic and permanent sparing. Sparing is used in combination with all protection types, including RAID 1, RAID 5, and RAID 6. Volumes with SRDF connectivity may also take advantage of sparing. Sparing offers the following benefits: Increases protection of all volumes in the event of a second drive failure in a RAID group. Automatically activates the spare volume without interruption prior to loss of access to a potentially failing volume. Ensures that the spare copy is identical to the original copy. Increases data availability of all volumes in use without loss of any data capacity. Is transparent to the host and requires no user intervention. EMC Symmetrix systems are self-diagnostic and proactive on parts replacement before an actual failure occurs. The sparing algorithms are designed to work in the same manner. Enginuity determines when a Applied Technology 4

5 physical volume is about to fail and begins the appropriate sparing operation. The Symmetrix system makes this decision based on error statistics maintained by its directors, the intelligent drive microprocessor self-testing information, and an active error checking system. This white paper describes the functionality of both dynamic and permanent sparing. Also discussed are the benefits and considerations of these processes, as well as the spare drive configuration rules. This white paper addresses Symmetrix DMX-3 and DMX-4 systems running Enginuity 5772 and later. The information in this paper is based on current microcode behavior and is subject to change. EMC is continuously improving and modifying operational functionality of all features based on testing and events experienced in the field. We are always searching for better ways to support our customers. Audience This white paper is intended for technology professionals who configure and support Symmetrix systems. It is also intended for operations and data center personnel who plan, install, and configure Symmetrix storage arrays in their IT environments. Terminology The following are explanations of terms that will be commonly used throughout the paper: Business Continuance Volume (BCV): Specially tagged logical volumes manipulated by using TimeFinder/Mirror commands to create copies of standard volumes. Uses for BCV data copies include backup operations, restore operations, decision support operations, and application testing. Note: In many situations it may be more appropriate to use clones or snaps to create local point-in-time copies. Please discuss with your local EMC Service Representative to decide what is best for your environment. Data Drive: A physical drive that has hypervolumes allocated on it that may be accessible to the host. Hypervolumes on two or more physical drives are used to form logical volumes. Dynamic Sparing: A function that runs within Enginuity that provides temporary data protection in the event of a physical drive failure. Hypervolume: A single physical drive may be split into two or more hypervolumes. Each hypervolume is grouped into separate logical volumes. The logical volumes may also be of different protection schemes, including RAID 1, RAID 5, RAID 6, and unprotected volumes. Note: Unprotected volumes have only one instance of the data. This protection method was typically reserved for short-term (for example, daily) backups (BCVs) or test data. Failure of an unprotected BCV could cause significant delay in backups or other procedures such as database consistency checks and quality assurance operations that leverage secondary copies of production data. TimeFinder/Mirror used in emulation mode and TimeFinder/Clone enable RAID 5 logical volumes to be used with the BCV attribute. Please discuss with your local EMC Service Representative to decide what is best for your environment. EMC requires an approved RPQ for unprotected standard volumes. Mirror Positions: Symmetrix Enginuity allows four logical mirror positions for each logical volume. RAID 1 and RAID 5 volumes occupy two mirror positions. RAID 6 volumes occupy only one mirror position. Other Enginuity features may occupy additional mirror positions of a logical volume, such as TimeFinder/Mirror and SRDF. When a dynamic spare is invoked to a logical volume, it will occupy a mirror position of the logical volume. Likewise, the logical volume must have an available mirror position in order for the dynamic spare to be successfully invoked. Applied Technology 5

6 For example, a RAID 1 volume with one SRDF mirror will have three mirror positions occupied. There will be one free mirror position. If a dynamic spare is then invoked to this volume, it will take up the fourth and final mirror position. Not Ready Spare: A spare drive that was previously a data drive and was replaced by permanent sparing. The drive will have a not ready flag set that will prevent it from being used until replaced with a new drive. Permanent Sparing: An automated, self-healing process that runs on the service processor to permanently replace a failing drive with a spare drive through a configuration change. Permanent sparing can also be thought of as a permanent replacement. Spare Drive: A physical drive that is not user-accessible and is reserved for use by dynamic and permanent sparing in the event of a drive failure. Standard Volume: Devices configured for normal Symmetrix operations. Standard volumes are typically the source volumes in a TimeFinder session with a BCV volume, unless there is a restore operation in progress. Dynamic sparing Dynamic sparing is a function running within Enginuity that provides incremental protection against failure of a second drive during the time a drive is taken offline and when it is ultimately replaced and resynchronized. Dynamic sparing is used in combination with RAID 1, RAID 5, and unprotected volumes. Dynamic sparing is not currently supported for RAID 6 or Virtual Provisioning data volumes. When a dynamic spare is invoked against a RAID 1 or mirrored volume, the entire contents of the failing volume will be copied to the dynamic spare. If the drive fails before the spare completes the copy, the remaining data will be copied from the other mirror. When a dynamic spare is invoked against a RAID 5 volume, only the contents of the failing member will be copied to the dynamic spare. If the drive fails before the spare completes the copy, the remaining data will be rebuilt from the members of the RAID group onto the spare. When the copy is complete, the spare becomes a full member of the RAID 5 group with the ability to perform XOR calculations. Only one dynamic spare is required to protect the RAID 5 group. Process The Symmetrix system makes sparing decisions based on error statistics maintained by directors, the intelligent drive microprocessor self-testing information, and active error checking systems. If the Symmetrix dynamic sparing algorithms determine that the number of errors occurring on a drive is excessive and that a hard failure is probable, a request for an available spare is created in global memory. The available spare must be of the same block size (512 or 520) as the failing drive. Drives with 520 block size are typically only used by iseries systems. The Symmetrix system dynamically copies all data from the failing drive to one or more available spares. The Symmetrix system notifies the EMC Customer Support Center of the event. The Symmetrix system continues to process host I/O requests at the highest priority while the copy operation takes place to minimize the effect on performance. After the failed drive is replaced, the Symmetrix system dynamically copies the contents of the spare to the new drive. The spare remains in use until the copy completes. When the copy has completed, the spare is released from the data drive and becomes an available spare again. Figure 1 gives a high-level illustration of the dynamic sparing process: Applied Technology 6

7 1) The first mirror (M1) begins to fail. M1 M2 2) Dynamic spare invokes to the next available mirror position (M3). Spare M1 M2 M3 3) Data is copied to dynamic spare. If the drive fails, data will copy from the other mirror. xm1 M2 Spare M3 4) The M1 drive is replaced and data is copied to the new drive. Data Flow Spare M1 M2 M3 Data Flow 5) The dynamic spare is released. M1 M2 Figure 1. Dynamic sparing process Benefits The ultimate benefit of dynamic sparing is data protection. The physical replacement of the affected drive can be performed while the logical volumes remain protected. Dynamic sparing resynchronizes a new drive with the dynamic spare after repair of the defective device is complete. In the event that a second mirror or member of a RAID group fails before the original drive has completed rebuild, the dynamic spare will protect the RAID group. The Symmetrix system also continues to process host I/O requests at the highest priority while the copy operation takes place. The size, speed, and location of the spare drive are not limiting factors. Multiple smaller spares can be used to protect a larger failing drive. A spare that is of a different speed than the failing drive will be allowed to provide the data protection, although performance may be temporarily affected. Dynamic sparing is performed at a logical level, meaning that if the dynamic spare is not able to invoke to one logical volume on a failing data drive for some reason, the dynamic spare will still be able to invoke against the rest of the logical volumes on the failing drive. For example, if only one of the logical volumes on a failing data drive does not have any free mirror positions, the dynamic spare will still be able to invoke to the rest of the logical volumes on the drive. Dynamic spares will not invoke against all physical drives during a fibre loop failure. The system will call home to the EMC Customer Support Center and support personnel will investigate. If a specific drive is found to be at fault, a dynamic spare will then be invoked manually to the faulty drive only. This reduces the need to remove dynamic spares from multiple drives that are not faulty. Applied Technology 7

8 Considerations Dynamic sparing results in two copy processes for each hypervolume on the physical: one copy to the dynamic spare, and another copy to the new drive after replacement. Back-end configuration changes are limited while a dynamic spare is invoked. Configuration changes that affect the spared drive are not allowed. One example is adding or deleting volumes from a physical drive that has a dynamic spare invoked. An invoked dynamic spare will temporarily occupy a mirror position of the volume it is invoked against. This may affect other Enginuity features, such as TimeFinder/Mirror, Dynamic SRDF, or SRDF/Star, which also occupy a mirror position of the standard volume. For example, if a RAID 1 standard volume with SRDF protection has a dynamic spare occupying the fourth mirror position, then a BCV could not be established to the volume. This could cause a delay in backups. This conflict can be mitigated through the use of TimeFinder/Mirror in emulation mode or TimeFinder/Clone, which do not occupy a mirror position of the standard volume. The configuration file settings described next provide other ways to avoid mirror position conflicts. Configuration file settings The configuration file has several settings that are used to allow or prevent invoking a dynamic spare against specific types of logical volumes. These settings may also help prevent mirror position conflicts. The options include: RAID 1 volumes SRDF source volumes SRDF target volumes Volumes that currently have a BCV attached Volumes that have a history of TimeFinder operations Allow dynamic spare to invoke into the fourth mirror position Please discuss these options with your local EMC Service Representative to decide which settings are best for your environment. Permanent sparing Permanent sparing is an automated, self-healing process that runs on the service processor to permanently replace a failing drive with a spare drive through a configuration change. Permanent sparing can also be thought of as a permanent replacement. The Symmetrix system will call home to notify the EMC Customer Support Center of this event. Permanent sparing is used in combination with all protection schemes. A dynamic spare may be used as part of the permanent sparing process, for example, if the failing drive contains unprotected volumes. Unprotected volumes have only one instance of the data so a dynamic spare will be required to provide a temporary copy of the data. Process Permanent sparing can be enabled or disabled from the Symmetrix configuration file. When Symmetrix Enginuity detects that a drive is about to fail and permanent sparing is enabled, a dynamic spare will immediately invoke to any unprotected volumes on the affected drive and begin copying data. However, the request for dynamic spare for locally protected volumes is initiated with a 10-minute delay. This 10-minute delay allows time for the permanent sparing process to begin in case there is another process is running on the service processor. When permanent sparing starts the dynamic spare request will Applied Technology 8

9 be cancelled. If permanent sparing does not begin within the 10-minute window, a dynamic spare will be invoked to all logical volumes on the failing drive. Once the permanent sparing process begins, it will look for a spare drive of the same block size, capacity, and speed in a good location to permanently replace the failing drive via configuration change. The process identifies a good location using the following rules: No RAID 1 mirror or RAID 5 group member on the same loop No more than two members of the same RAID 6 group on the same loop Drives that contain vault volumes are candidates for permanent sparing if: On a DMX-3, there is an available spare on the same primary loop. The vault volume must stay on the same loop on the same disk director processor On a DMX-4, there is an available spare on the same disk director processor. The vault volume can move to a different loop on the same disk director processor Note: If the process cannot identify a spare in a good location, the dynamic sparing process will take place for RAID 1, RAID 5, and unprotected volumes. When a suitable spare is identified, the permanent sparing process loads a new configuration file in which all the logical volumes initially configured on the failing drive are now configured on the selected spare (new) drive. The configuration change typically takes a few seconds to complete, during which time the configuration will be locked. The configuration change will take place as soon as all of the volumes on the failing drive have a valid local copy or can be rebuilt through the RAID group. If dynamic sparing was invoked against unprotected volumes, the configuration change will take place as soon as the copy to the dynamic spare is complete. Data will then be copied or rebuilt onto the new drive. The Symmetrix system continues to process host I/O requests at the highest priority to minimize any effects on performance. The configuration is not locked during this rebuild process. The failed drive becomes a not ready spare in the spare pool and can be replaced at a later date. There should be other spares available for other drive failures. The drive will be marked not ready to prevent use as either a dynamic or permanent spare. The not ready state also prevents the drive from being introduced onto the fibre loop in the event of system IML, disk director IML, or power cycle. The not ready setting will be removed once the drive is physically replaced, at which time it will again become an available spare. Figure 2 gives a high-level illustration of the permanent sparing process using RAID 1 as the example. Permanent sparing of RAID 5 and RAID 6 volumes is identical in that the configuration change will move the data volumes to the spare drive. The manner in which data is built onto the new location depends on the protection scheme of the logical volume. A physical drive may be configured with logical volumes of different protection schemes. Applied Technology 9

10 1) The first mirror (M1) begins to fail. A suitable spare is identified. M1 M2 Spare 2) The failing drive is set to not ready. xm1 M2 Spare 3) A configuration change is performed to logically replace the failed drive with the spare. 4) Data is copied to the new drive. The former spare drive is now in the M1 position. Configuration Change M1 M2 M1 M2 Data Flow Spare x Spare x Figure 2. Permanent sparing process Benefits Permanent sparing allows the system to get back to N+1 protection status in the shortest possible time frame. The process runs unattended upon detection of a drive failure. The amount of time that a data drive remains not protected in the system is reduced. The amount of copy processes is also minimized; locally protected volumes require only a single copy process. Thus, the amount of time required to rebuild and copy back the data from the failed drive is significantly decreased. Permanent sparing does not require an available mirror position for locally protected volumes. This avoids potential conflicts with other Enginuity features. Replenishment Process for Permanent Spares In addition to the previous benefits, Symmetrix DMX-3 and DMX-4 systems that have permanent sparing enabled and are running Enginuity or or later can take advantage of an optional feature called Replenishment Process for Permanent Spares. This feature allows for spare drives that have been replaced by permanent sparing to not be called out for immediate replacement. The Replenishment Process has the added benefit of allowing the actual physical replacement of the failed drive to be scheduled for a time slot that is convenient for the customer. Other spares will be available should another drive fail before the not ready spare is replaced. Furthermore, the actual replacement time of a spare drive is significantly less than that of a data drive since no copy or rebuild process is required. All systems that use permanent sparing can take advantage of these benefits. Applied Technology 10

11 With the Replenishment Process enabled, a successful permanent sparing operation will generate a call home creating a service request for the event. As long as all drives in the system still have permanent spare coverage, the service request will be automatically closed. At the point when a permanent sparing operation causes any drive in the system to no longer have permanent spare coverage, the system will generate a log file named DDS.log that contains all of the specific details necessary for the replacement of all spare drives that were previously used by permanent sparing. The system will call home and generate a service request that contains the log file, which will be dispatched to an EMC Customer Engineer (CE). The CE will contact the customer to schedule a convenient time to conduct the replacement of multiple drives during a single trip to the customer s site. If for any reason permanent sparing is unable to run on a failing drive, a dynamic spare will be invoked against the failed drive if possible, and the system will call home to alert the EMC Customer Engineer for same-day replacement. To enable the Replenishment Process for Permanent Spares, check the Enable Deferred Service flag in the Symmwin Site Information window. EMC Certified Data Erasure for Symmetrix Disks Data Erasure is a feature that is only available on Symmetrix systems that implement permanent sparing. Data Erasure erases data on a physical medium where the granularity is an entire physical drive. The erasure can vary between a single pass write of data to the drive, all the way to multi pass erasure using complex algorithms as used in DoD M. The intent of secure disk erase is to remove all recoverable user data from a given physical drive, or report that the attempt to remove the data was unsuccessful. If the EMC Certified Data Erasure for Symmetrix Disks Service license has been purchased, the customer will receive a certificate of completion for all disks successfully erased. Nonerasable drives will be left with the customer for disposition. Data Erasure is only able to run on a not ready spare upon successful completion of permanent sparing. This prevents a drive that is visible to the host from being exposed to Data Erasure. Data Erasure is not currently supported for Flash drives. More detailed information on EMC Certified Data Erasure for Symmetrix Disks is available on Powerlink or through your local EMC Service Representative. Note: The DMX-3 spare rules may not provide enough spare drives to maximize the use of Data Erasure. Therefore it is highly recommended and considered best practice to configure additional spare drives when implementing Data Erasure on DMX-3 systems. The DMX-4 sparing rules should be used as a guideline. Both the DMX-3 and DMX-4 spare rules are discussed in following sections. Considerations Not all drive failures may be candidates for permanent sparing. One example is if a suitable spare cannot be identified. A failing drive cannot be permanently spared by a spare drive of a different speed, block size, or capacity. Permanent sparing will also not be able to run if the configuration is already locked. Please note that in both of these situations the failing drive will be protected by dynamic sparing. Permanent sparing will not run if any unprotected volumes on the failing drive fail to copy to the dynamic spare. In this case, the system will call home to the EMC Customer Support Center and support personnel will investigate. Permanent sparing also will not run if there are multiple drives not ready on a single DA processor. The cause of the failure may be something other than the drive itself. Again, the system will call home to the EMC Customer Support Center and support personnel will investigate. Applied Technology 11

12 Configuring spares When building a configuration for a new system before any logical volumes are present, the required amount of physical drives will be automatically marked as spares. The spare drives will be evenly distributed among the available disk directors. Local EMC Service Representatives can override this algorithm and manually decide on the physical placement of the spare drives. The same conditions apply when adding new drives (spare drives and data drives) to an existing system. If adding only spare drives to the system and there are multiple open slots to choose from, the disk map in the configuration file, described on the following pages, will be utilized by the EMC Service Representative to determine which physical location(s) will give the best permanent sparing coverage. Permanent spare coverage There are two ways to determine the permanent sparing coverage for each data drive. Both methods are currently available only to EMC personnel as access to SymmWin, the graphics-based tool for configuring and monitoring a Symmetrix system, is required. Please contact your local EMC Service Representative for assistance with determining permanent spare coverage. Configuration file drive map The drive map in the Symmetrix configuration file has an option to view permanent sparing coverage. When selected, each drive in the map will display a number indicating how many spare drives it can use for permanent sparing. When a specific drive is selected, the acceptable spares are indicated. The drive map is based on the physical location of the spare drives and does not take into account if any spares are set to not ready. Query Spare Pool Status script The Query Pool Status script queries the current status of the system to provide a detailed report of the status of the spare drives and the permanent sparing coverage of the data drives. For each data drive configured in the system, the script will report which spare is its first choice for permanent sparing coverage when available. The script will also report which spare is the current choice. These two fields will typically be the same unless the first choice spare is currently unavailable. The script also displays the following information: Total data drives Total spare drives Number of data drives that have permanent sparing coverage Number of data drives that do not have permanent sparing coverage Physically ready spare drives Physically not ready spare drives Spare drives that have the not ready bit set Spare drives that are dynamically invoked All of this information will then be stored in a log file on the service processor. Spare rules The rules for the numbers and types of spares required for each system were decided after careful analysis of field data. These rules are designed to minimize risk by providing multiple spares in various locations for each type of data drive configured. The system s current configuration as well as expansion potential is Applied Technology 12

13 taken into account. Spares must be configured for each of the system s drive types (capacity, speed, and block size). The DMX-4 and the DMX-3 rules differ slightly. Analysis of DMX-3 systems showed that many systems did not have a sufficient number and/or types of spares to fully support both dynamic and permanent sparing. Thus, DMX-4 sparing rules were increased to a level that provides spare coverage even if there are spare drives unavailable for some reason. For example, a spare drive or drives may be set not ready, may be already in use, or may be faulty. The rules were not increased for DMX-3 because a change of restrictions to existing systems may have caused upgrade complications. The rules for both DMX-3 and DMX-4 are considered to be the minimum and additional spares can be configured. Spare drives on every loop for every type of data drive on the loop will give the best possible permanent sparing coverage. DMX-4 spare rules Spare drives are required for every DMX-4 system. SymmWin prevents creation of DMX-4 configurations that do not contain the required numbers and types of spare drives. A valid pool of spares for each drive type must be available. The amount and types of spares required for DMX-4 systems are calculated as follows: DMX-4 or DMX with four directors Two spare drives for every 100 physicals of each drive type Minimum of eight spare drives for the entire system DMX with two directors Two spare drives for every 100 physicals of each drive type Minimum of four spare drives for the entire system DMX-3 spare rules The EMC ordering system will calculate the minimum number of spares that must be configured in each DMX-3. This ordering system will also automatically select the types of the spare drives. At least two spare drives will be configured for each type of drive in the Symmetrix. However, the remainder of spare drives will be defaulted to the largest/slowest type of drive, which may not be the type of drive that is most prominent in the configuration. This could severely limit the chances of successful permanent sparing. Configuring spare drives of the largest/slowest drives in the system was the standard prior to the introduction of permanent sparing. This guaranteed that any spare drive could protect any failing data drive with dynamic sparing. This method was carried over in part when permanent sparing was first introduced with the idea that the larger spares would be able to dynamically protect any drive in the system if the smaller spare drives were unavailable. The method of configuring larger/slower spare drives became unnecessary with dynamic sparing improvements in Enginuity In this code dynamic sparing acquired the ability to use multiple smaller spares to protect a larger failing drive. This functionality increased sparing flexibility with the benefits of improving permanent sparing coverage for smaller capacity drives and eliminating the need to have a dominant amount of large capacity spare drives. Therefore, for the DMX-3 it is recommended to configure spare drives proportional to the amount of each type of drive configured in the Symmetrix. Calculate the number and types of spares for DMX-3 systems as follows: Two spare drives for each type of drive in the system One additional spare drive for every 100 drives or portion thereof for each type of drive in the system Applied Technology 13

14 Special DMX-3 sparing considerations Additional spare drives may be required in certain DMX-3 configurations. In the case of RAID 5 (3+1) or RAID 6 (6+2) on a single-director pair DMX-3 950, and RAID 5 (7+1) or RAID 6 (14+2) on both the two-director pair DMX and the DMX , one spare drive needs to be configured on every loop in order to fully implement permanent sparing. Configuring fewer spares would mean that only those drives with a spare on the same loop could be permanently spared. Furthermore, since only one spare is available for each drive, then the spare would need to be replaced before permanent sparing again fully protects the entire system. In these special circumstances the option to allow a number of drives to fail and maintain the ability to permanently spare all drives is not available. Configuring two or more spares per loop will alleviate this issue. Flash drives and sparing Flash drives are available for DMX-4 systems. Flash drives support both dynamic and permanent sparing using Flash drive spares. Symmetrix Enginuity will block sparing between Flash drives and magnetic disk drives to avoid potential performance issues. One Flash drive spare must be configured for every 32 data drives, or portion thereof. If more than one capacity of Flash drives is installed, the Flash drive spare must be equal to the highest capacity Flash drive. This will provide dynamic sparing coverage for all Flash drives. However, permanent sparing of Flash drives requires a spare of the exact size as the failing drive. The system spare drive requirements are not affected by the configuration of Flash drives or Flash drive spares. The system s required amount and types of spares are solely determined by the configured magnetic disk drives. Flash drive spare requirements are determined independently. Conclusion Today s mission-critical environments require a high-end storage solution that guarantees uncompromised levels of availability. This solution must include a robust architectural design to withstand any and all potential failures without impacting data availability. High-end availability is more than just redundancy; it means nondisruptive operations and being always online. When a drive failure occurs, the respective RAID group will be temporarily exposed. Sparing reduces the time that the RAID group will be exposed. The sparing operations are transparent to the host and require no user intervention. While sparing operations take place host I/O requests continue to be processed at the highest priority to minimize the effect on performance. Sparing is one of the advanced data availability features that have elevated customer expectations for highend storage to a new level making Symmetrix the ideal choice for critical applications and 24x7 environments demanding uninterrupted access to information. Applied Technology 14