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EMC Global Education

PowerPath Foundations – IMPACT Course Description

e-Learning

This foundation level course provides participants with an understanding of how EMC PowerPath is used to improve performance and increase availability as part of EMC’s Technology Foundations. This course is part of the EMC Technology Foundations curriculum and is a pre-requisite to other learning paths.

Course Number:

MR-WP5-PWRPFD

Method: IMPACT Duration: 1 hour

Audience This course is intended for any person who presently or plans to:

• Educate partners and/or customers on how EMC PowerPath can maximize availability and performance in a Symmetrix and CLARiiON-based storage infrastructure

• Provide technical consulting skills and support for EMC products

• Analyze a Customer’s business technology requirements

• Qualify the value of EMC’s products

• Collaborate with customers as a storage solutions advisor

Prerequisites Prior to taking this course, participants should have completed the following Courses:

• None

Course Objectives Upon successful completion of this course, participants should be able to:

• Discuss the features and benefits of PowerPath in an open systems host environment

• Explain how PowerPath achieves transparent recovery

• List the environments that PowerPath supports

Modules Covered This course includes one module:

• PowerPath Foundations

Labs Labs reinforce the information you have been taught. The labs for this course include:

• None

Assessments This course includes a self-assessment quiz to be conducted on-line via KnowledgeLink, EMC’s Learning Management System.

Copyright © 2004 EMC Corporation. All Rights Reserved.

PowerPath Foundations, 1

11EMC Global Education© 2004 EMC Corporation. All rights reserved.

PowerPath Foundations

Welcome to PowerPath Foundations.EMC PowerPath is a server-resident software product that provides application availability when a SCSI I/O channel or Fibre Channel path fails. It also improves a servers ability to handle heavy storage I/O loads.



EMC Global Education© 2004 EMC Corporation. All rights reserved. 22

EMC Technology Foundations

EMC Technology Foundations consists of AutoIS, SAN products, NAS products, and Storage Platforms, as well as advanced storage management software.EMC Technology supports the portfolio of end-to-end services designed to help accelerate the implementation of Information Lifecycle Management (ILM).ILM uses EMC Technology to enable organizations to better and more cost-effectively manage, properly protect, achieve compliance and improve the availability of their business information in a way that ties varying information usefulness to business goals and service levels.

This course describes PowerPath and its features that contribute to EMC Technology Foundations.




Course Objectives

After completing this course, you will be able to:Discuss the features and benefits of PowerPath in a server environmentExplain how PowerPath achieves transparent recovery List the environments that PowerPath supportsDescribe the management functions available using the GUI and CLI interface

These are the objectives for this training. Please take a moment to read them.




PowerPath - Introduction

Server based software– Most Unix Hosts– NT/W2K

Each Logical Volumecan be configured with up to 32 paths from the storage system to the host– Automatic and non-disruptive

path failover eliminates HBA, cable,channel director or ESN component as single point of failure

– Automatic load balancing over all available paths can improve performance

Hos

tH

ost

Com

pone

nts

Com

pone

nts Host ApplicationsHost Applications

Symmetrix Symmetrix and CLARiiONand CLARiiON

Device LayerDevice Layer

EMCEMC22 PowerPathPowerPath

PowerPath is server-resident, performance and information availability enhancing software solution. It combines multiple path I/O capabilities, automatic load balancing, path failover, and logical volume management functions into one integrated solution.

Automatic: PowerPath uses sophisticated algorithms to increase application I/O rates through Symmetrix and CLARiiONwith automatic data path load balancing allowing for greatest efficiency and throughput. PowerPath’s volume manager capability simplifies disk administration tasks to further reduce total cost of ownership through high-level commands that hide storage complexity. It automatically manages workloads and volume expansion.Automatic path failover—By taking advantage of all paths, PowerPath increases the ability of server access to data on the Symmetrix and CLARiiON arrays. PowerPath moves I/O workloads across multiple paths to ensure fastest possible I/O speed. Non-disruptive: PowerPath provides users application to storage automation, seamless data migration, and automatic import of Volume Groups, and simplifies the growth of logical volumes while application remain online. PowerPath optimizes server and data path utilization by avoiding downtime.Optimized: By leveraging your server, SAN, and storage assets, PowerPath maximizes your investment by increasing storage utilization.Dynamic load balancing—When the host needs access to data on the Symmetrix or CLARiiON, PowerPath uses the quickest and easiest route or path to access the data. PowerPath supports several load balancing policies compared to other vendors. In the rare instance of a path failure, PowerPath will re-issue I/O to an alternate path maintaining data availability and ensuring information access. PowerPath offers policy-based dynamic path management that accelerates information access and provides high availability.




PowerPath Features

Automatic path failover Dynamic load balancingLogical Volume Manager Automatic detect and restore of failed componentsOnline configuration and management Common HBA driver support across Symmetrix and CLARiiONWrite throttlingConsistency Group support (Symmetrix only)

Other attributes, in addition to Automatic path failover and dynamic load balancing include:Integrated Logical Volume manager that provides a logical abstraction of storage.Automatic detect and restore of failed components— For example, if a cable is mistakenly dislodged, PowerPath will take all existing I/O that was going down that particular path and re-route it to another active path. Once the cable is re-attached, another feature of PowerPath, called Auto Restore, automatically restores path access permitting data flow down the path again. Online configuration and management—You can have some paths active and some passive (or "standby"). This means that the passive paths will only be used if the active paths fail. A user can segment applications; for instance, segment logs from table spaces, or segment one application from another. For example, in an eight-path configuration running two applications, you can have each path active for one application and passive for the other. PowerPath allows the user to make configuration changes while applications remain online, meaning you can reconfigure the load balancing policy, take paths offline and online, hot repair, etc. PowerPath also supports common HBA driver connectivity on the Symmetrix and CLARiiON, which means that one host can share the same HBA to both Symmetrix and CLARiiON arrays. PowerPath also supports write throttling, which is a way to way to minimize or eliminate database read timeout errors, by throttling the write so that a read can be processed.For a group of Symmetrix devices, PowerPath will maintain the integrity of a database distributed across multiple SRDF units. PowerPath also supports the ability of booting from a mirrored R2 device in an SRDF configuration.




Path Fault without PowerPath

In most environments a host will have multiple paths to the Storage SystemVolumes are spread across all available pathsEach volume has a single pathHost adapter and cable connections are single points of failureWork load not balanced between all paths Storage

Host Application(s)

SD

HBA

SD

HBA

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HBA

SD

HBA Host BusAdapter

SCSIDriver

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GE Interconnect

Topology

Without PowerPath or another path failover software package, the loss of a channel (red dotted line) means one or more applications may stop functioning. This can be caused by the loss of a Host Bus Adapter, Symmetrix Channel Director, CLARiiON Storage Processor, FC-AL hub, or a failed (kicked) cable. In a standard non-PowerPath environment, these are all single point of failure. In this case, all I/O that was heading down the path highlighted in red is now lost, resulting in an application failure and the potential for data loss or corruption.




PowerPath Configuration

All Volumes are accessible through all pathsApplications direct IO to PowerPathPowerPath directs I/O to optimal path based on current workload and path availability

Host Application(s)

HBA HBA

SD SDSD

HBA Host BusAdapter

SCSIDriver

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PowerPath

Without PowerPath, if a host needed access to 40 logical volumes, and you had four host bus adapters, you would most likely set it up so that 10 volumes were presented to each host bus adapter. With PowerPath, you would set it up so that all 40 volumes could be “Seen” by all four host bus adapters. This would require:

• The storage system be configured to present the volumes to multiple front-end ports• The SAN fabric is zoned • Access control through Volume Logix or Storage Logix• HBA binding (if needed)

If set up properly, each Logical volume would be presented multiple times. For example, in a Solaris environment, if SLV 001 is presented to four different front-end directors, and the fabric is zoned properly, that single volume may actually have four different logical device file names. I.e. /dev/dsk/c1t0d0, /dev/dsk/c2t0d0, /dev/dsk/c3t0d0, /dev/dsk/c4t0d0. While this may be somewhat confusing the first time you see this, PowerPath understands that the four device files actually al point to a single volume and will direct the I/O appropriately.




Transparent Recovery with PowerPath

If a host adapter, cable or channel director/Storage Processor fails, the device driver returns a timeout to PowerPathPowerPath responds by taking the path offline and re-driving I/O through an alternate pathSubsequent I/Os use surviving path(s)Application is unaware of failure

Host Application(s)

HBA HBA

SD SDSD

HBA Host BusAdapter

SCSIDriver

Storage

SER

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Topology

SD

HBA

PowerPath

PowerPath provides transparent recovery in response to a path failure. PowerPath will ensure that the I/O heading down the failed path is re-directed (keeping in mind load characteristics and priorities) to the most suitable alternative path. Unlike other path management products that will failover all I/O to the next available path (overburdening the path), PowerPath uses intelligence to decide where to failover the I/O.Because PowerPath spreads the additional load across the remaining channels, the system will continue to operate, however, in a “degraded” mode. This takes the pressure off the staff to make corrections immediately. Since some O/S servers do not support hot plugging of boards, so the replacement of an HBA may require bringing the server down, PowerPath allows the IT staff to schedule the repair at a convenient time.If you are concerned with degraded mode behavior, the PowerPath GUI provides the ability to disable and re-enable channels on the fly so you can test the performance sensitivity of your configuration.Simple channel failover products work in pairs, i.e., one channel is the backup for the other. In the diagram, two pairs of channels would be set up. If one channel fails, all load for that channel goes to its “twin,” essentially doubling its load. With PowerPath, the load is spread across the three remaining channels. The objective is to make channel failover a non-issue. There are low cost products that provide two channel failover. By making queue management the issue, PowerPath is not competing with channel failover products and establishes the value of the PowerPath solution. The ability to compensate for a lost channel by spreading the I/Os across multiple channels instead of dumping all of it on one requires multi-channel support.




I/O without PowerPathQueues Out of Balance

I/O performance is partially based on queue lengthOptimal performance cannot be achieved unless work is balancedI/O workload across HBAs is seldom balancedAny any point in time some paths may be idle while other paths have multiple I/O operations queued

Host Application(s)

Storage

HBA HBAHBA Host BusAdapter

SCSIDriverSD SDSD SD

HBA

Request

Request

Request

Request

Request

Request

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Request

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The server views each disk resource to be on a single path. EMC technical personnel set up the configuration of the storage system to spread I/O load across the paths. Each application is set up with its own storage. The storage is allocated to Channel Directors or Storage Processors based on expected data requirements of the applications. This setup is done based on snapshot measurements, guesstimates of average loading and predictive loads, and the experience of the EMC experts. This diagram depicts a snapshot of the system at a moment in time. The depth of the I/O queues is very unbalanced. Host applications sitting on top of deep queues are not getting the data they need. If this was the average loading, the System Administrator would reconfigure the system to balance the load better. In any system, there will be points in time when the load is unbalanced due to one application receiving heavy I/O requirements. In this instance, two of the applications are currently causing high I/O traffic. At this point, two channels are overloaded (depicted by the red disks on the outside, and the pending request stack) while two other channels are lightly loaded. In a while, the requests will have been handled and the system will return to a more balanced load. In the meantime, the applications are being “data starved” and the users or applications are experiencing less than optimal performance.




I/O with PowerPath Queues in Balance

PowerPathdynamically balances workload across all available pathsPowerPath will provide greatest improvement in performance in environments where the workload is not balanced– Workloads are

seldom balanced– Workloads

dynamically change

Host Application(s)

Host BusAdapter

SCSIDriverSD SDSD SD

HBA HBAHBA HBA

Request

Request

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Request Request

Request

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PowerPath

InterconnectTopology

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With PowerPath in the system, applications transparently access PowerPath instead of the SD (SCSI driver) devices. PowerPath allocates the requests across the available channels, reducing bottlenecks and improving performance. This diagram shows a similar snapshot with PowerPath using multiple channels to minimize the queue depth on all channels.Since the Channel Directors or Storage Processors are writing to cache and not to disks, any Channel Director/Storage Processor can handle any request. This allows PowerPath to constantly tune the server to adjust to changing loads from the applications running on the server. PowerPath improves the performance of the server, enabling it to make better use of the storage. This results in better application performance and less operational resources spent on the care and feeding of the system, and more (financial) value from your server investment.PowerPath does not manage the I/O queues; it manages the placement of I/O requests in the queue.




Performance Tuning and Active / Standby Channel Policies

CacheCache

SCSI Driver

Host BusAdapter

ChannelGroup

ChannelGroup

Standby PathsApplication B

Available bandwidth can be segmented per application

SD SDSD SD SD SD

HBA HBAHBA HBA HBA HBA

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SCSI Driver

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Host Application A Host Application B

PowerPath provides configuration flexibility that allows you to define some paths active and some passive or "standby". This means that the passive paths will only be used if the active paths fail. You can dynamically add and remove paths. For example, if you had eight paths partitioned into four for one application and four for another, and one application needed more bandwidth, say during batch, you can, through scripts, add more paths during the batch run and take them away when you're done. The actual command is powermt set mode=standby/active adapter=adapter# dev=integer/all

Here are examples of two practical applications of this:If you have two applications on a server that are both very busy, but you don’t want the activity of one application to

impact the performance of the other application, you could set up the two applications to have their own sets of channels. I/O for those two channels would be balanced among the channel groups only, and if one application was very busy, the other application would not be impacted from an I/O standpoint. In the event of all the defined channels for one Channel Group failing, I/O would be re-routed to the other Channel Group’s channels.

If a host has two types of channels, for example, Fibre Channel and SCSI, you would have to segregate channels by channel type. It is not possible for PowerPath to load balance an application across both Fibre Channel and SCSI together, but by using channel segregation, a user can set up the environment so that PowerPath would load balance across the SCSI channels for one set of devices and Fibre Channel for the other set of devices. Then, in the event of a failure, all the devicesdefined to the failed paths would use the other available paths for I/O, because they were standby paths for those devices.




PowerPath Load Balancing Policies

Symm_opt / CLAR_opt (default) – I/O requests are balanced across multiple paths based on composition of reads, writes, and

user-assigned device / application prioritiesRound_Robin

– I/O requests are distributed to each available path in turnLeast_I/Os

– I/O requests are assigned to the path with the fewest number of requests in the queueLeast_Blocks

– I/O requests are assigned to the path with the fewest total blocks in the queueRequest

– Path failover onlyNo Redirect

– Disable path failover and load balancing (same as Transparent=on)– Default for Symmetrix when there is no license key; not supported on CLARiiON

Basic Failover – CDE-type functionality (SPA–SPB failover)– Default for CLARiiON when there is no license key; not supported on Symmetrix

The Symm_opt/CLAR_opt policies — Choose paths based on path load and logical device priority. Device priority is host based priority and not Symmetrix/CLARiiON level priority.Round Robin — I/O requests are assigned to each available path in rotation.Least I/Os — Load balance is based on the number of pending I/Os. I/O requests are routed to the path with the fewest queued requests, regardless of total block volume.Least Blocks — Load balance is based on the number of blocks in pending I/Os. I/O requests are routed to the path with the fewest queued blocks, regardless of the number of requests involved.Request — For native devices, this policy uses the path that would have been used if PowerPath were not installed. For pseudo devices, it uses one arbitrary path for all I/O. For all devices, path failover is in effect, but load balancing is not.No Redirect — Neither load balancing or failover is in effect. If this algorithm is set on a failed path and a native device is used, I/O errors will occur when I/O is directed to that path. This policy is valid only on Symmetrix arrays.Basic failover — Load balancing is not in effect. I/O routing on failure is limited to one host bus and one port on each Storage Processor. This policy is required for a non-disruptive upgrade. It protects against Storage Processor and back-end failures, but not against HBA or host loop failures. PowerPath is required to prevent HBA and/or host loop failures.You can change the policies from the powermt set policy command, or within the GUI on Windows.




Active-Active vs. Active-Passive

CLARiiON “back-end” employs dual-port disks, redundant FC-AL loops– All disks have

paths to both Storage Processors

CLARiiON LUNs are “owned” by one Storage Processor– One Storage

Processor services I/O for LUN

– Second path to LUN is passive

CLARiiON’s “LUN Ownership Model”

* Symmetrical Volume Access: volume accessible (active) on all paths

* Asymmetrical Volume (LUN) Access: LUN accessible (active) on one Storage Processor at a time

DirectorDirector

Symmetrix

Host Host

HBA HBA HBA HBA

Port Port Port Port

Switch Switch

SCSI disk

SCSI disk

SCSI disk

Director

Cache

StorageProcessor

CLARiiON

Cache

Host Host

HBA HBA HBA HBA

Port Port Port Port

Switch Switch

SCSI disk

SCSI disk

SCSI disk

StorageProcessor

CacheState Info

CMI

Architecturally, the Symmetrix supports an Active-Active access in that the same LUN can be presented and is accessible simultaneously through multiple front-end ports. The CLARiiON uses a LUN ownership concept where a LUN is “owned” by on storage processor at a time. In the diagram above, there are four available paths to the CLARiiON on this host.PowerPath enhances CLARiiON high availability with I/O path failover software. During normal operations, the host utilizes two high-speed data paths.In the event of a failure of a HBA, Storage Processor, or cable, PowerPath automatically reroutes traffic in seconds. By taking advantage of both paths under normal conditions, PowerPath increases the availability of the server to access data on the CLARiiON. All this is accomplished while the application remains online.




Failover Redefined with PowerPath

Use all active paths before doing a trespassWhen all active paths are exhausted, trespass on a LUN by LUN basisScope of LUNs failing over depends upon component– If there is a failure to an HBA or cable connecting the HBA to a

Storage Processor, hub, or switch, just the affected host’s LUNs fail over

– If there is a failure to a Storage Processor, all LUNs assigned to that Storage Processor will fail over if they are accessed

Failover occurs on a LUN-by-LUN basis. This helps avoid any unnecessary trespassing of devices that can cause possible performance issues and degradation.A multi-path failover will re-route the I/O through an alternate path to the LUN through the Storage Processor that is the primary owner. If the LUN is still inaccessible after going through all possible paths, a LUN trespass will occur. Trespass means that LUN ownership has been transferred to the alternate Storage Processor.The manner in which logical units fail will also depend upon the mode of failure within the data path. If there is a failure of the HBA, or anywhere between the server and the array, just the affected host’s devices would trespass over to the other Storage Processor, so they are accessible to the server through the alternate data path.If there is a Storage Processor failure, all devices assigned to that Storage Processor that received I/O from the requesting host will fail over to the surviving Storage Processor.




PowerPath Failover with CLARiiON

Path FaultHBA FaultStorage Processor Fault

CLARiiON

ApplicationApplication

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Request

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Request

PowerPath

SPSP--BBSPSP--AA


PowerPath protects the customers environment in case of a path, HBA, or Storage Processor failure. Path fault—If a path or a cable is accidentally bumped or pulled, PowerPath will automatically fail over the path to an alternate path. Once the path(s) have been restored, PowerPath automatically recognizes this and resumes sending I/O down those paths. HBA fault—If an HBA is lost at the front end, PowerPath will fail over the entire path to an alternate HBA. Once the HBA is brought back online, PowerPath recognizes this and resumes sending I/O through the alternate HBA.Storage Processor fault—If a SP-B fails (in diagram), PowerPath will stop all I/O that was currently headed from the server to SP-B and trespasses it over to SP-A; once SP-B is brought back online, PowerPath recognizes that SP-B is now available and resumes sending I/O down to SP-B.In the event of any of the above scenarios taking place, PowerPath ensures that applications remain continuously available.




PowerPath Load Balancing with CLARiiON

LUN Ownership Model with CLARiiON– Load balancing

• Load balancing and failover across active channels• Trespass to passive channels in event of all paths / Storage Processor

failover• Only on systems with two or more FC-SW (CLARiiON FC4700 and

above) ports per Storage Processor• For HBAs on active paths only• Not across Storage Processors

– Customer selects best algorithm for application requirements• CLAR_opt is usually recommended

Why active and passive paths? The reason—the CLARiiON LUN ownership model. The LUN ownership model means that only one Storage Processor can own a particular LUN at any one time. For the other Storage Processor to gain access to the LUN, it must take ownership of the LUN.Some people describe this as “active-passive” (on active-passive storage systems, “available“ paths are those paths leading to the active Storage Processor for each logical device). However, there can be many LUNs on a CLARiiON and each of these LUNs can be assigned to either Storage Processor. PowerPath can distribute the I/O for all logical devices over all paths shared by those logical devices, so all paths are equally burdened.PowerPath load balances I/O on a host-by-host basis. It maintains statistics on all I/O for all paths. For each I/O request, PowerPath intelligently chooses the least-burdened available path, depending on the load balancing and failover policy in effect. If an appropriate policy is specified, all paths in a PowerPath system have approximately the same work load. If no preference is given, PowerPath will choose what it thinks is the best method of load balancing.




PowerPath / CLARiiON (CDE Functionality)

Provides back-end failover, non-disruptive upgrades, and single HBA configuration support for single-attach hostsIs included in CLARiiON Utility KitAccess Logix is requiredNon-HA environment No multi-pathing capabilities

Application

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Request

PowerPath

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Fibre Channel Switch

CLARiiON

As a special case for non-high availability environments, PowerPath replaces CDE (CLARiiON Driver Extensions) functionality. It is provided on all CLARiiON arrays, and provides single HBA configuration support, back-end failover support, and non-disruptive upgrade support. This does not have any multi-pathing or load balancing capabilities and should not be used in a high availability environment.




PowerPath and HA Clusters

PowerPath enhances High Availability Cluster environments Increases application availability by eliminating the need to fail over when a path failure occurs Tested with leading HA cluster products– HACMP for AIX– HP MC ServiceGuard– Veritas FirstWatch– Sun Cluster– Qualix HA– NT MSCS

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Open Systems clustering technology manages application availability by detecting failures and restarts high availability applications on a surviving cluster node. The deployment of PowerPath in the cluster eliminates the application downtime due to a channel failure. PowerPath will detect the channel failure and use alternate channels so that the cluster software does not have to reconfigure the cluster to keep the applications running. PowerPath improves the availability of the applications running in the cluster.Many clusters are deployed to provide performance scalability (i.e., Oracle Parallel Server). PowerPath’s load balancing can help the customer maximize performance and get the greatest value from their cluster investment.PowerPath must work cooperatively with the cluster software. This does not require special code in the cluster, but does require PowerPath to work with storage management component of the cluster software. Validating PowerPath with the cluster software systems requires extensive knowledge and testing of the cluster software with PowerPath. The 2 key reasons customers invest in clusters is for application availability and system scalability. Most customers can achieve >99.5% uptime when deploying a cluster, as contrasted to a single computer server. PowerPath further adds to application uptime by greatly reducing the likelihood of a server failover due to a path failure.




Management

PowerPath provides GUI and CLI interface for online management– Performance and

status monitoring– Configuration– Channel director

and path maintenance

– Restoring path after repair

– Testing

When PowerPath is installed, maintenance, configuration and tuning can be performed online without interrupting the applications. When a path failure occurs, PowerPath disables the path. When the user has made a repair, a simple command line entry will enable I/O through the path. The applications will not be affected when re-establishing the path. All components in a Symmetrix are hot pluggable; however, the server does not handle the removal of a cable and channel director. Because the application has only one path to the device, the application will cease functioning. Further more, the server may have to be rebooted to re-establish communication on the channel. With PowerPath, the user can disable a channel causing I/O to use the remaining channels. The user can perform maintenance on the channel director without taking the application down, then, using PowerPath, re-establish the channel. Now every component of the Symmetrix is not only hot pluggable, it is hot serviceable.PowerPath provides tools to look at the behavior of the SCSI I/Os. You can monitor the amount of traffic on each queue and to each device. When PowerPath is installed, all configuration and maintenance is performed online, keeping the server available to do productive, revenue-generating work.PowerPath does not automatically reconnect because:

• When a channel has failed, you do not want to spend resources checking on it repeatedly• If a channel is “flaky,” you do not want to continue to try to use it while getting intermittent errors and forcing retries

across other channelsWhen a channel is lost, the server will continue to operate in a degraded state. How degraded it really is can be easily determined. Take a channel offline. If it causes a serious problem, you can return it to service in a few seconds. Additionally, suppose you have run out of slots on the Symmetrix; you want to add SRDF for disaster recovery and you need to add Remote Link Adapters (RLDs). You can see how the server will respond to having fewer SCSI channels available.




Course Summary

Key points covered in this course: PowerPath is server based software that provides multiple paths between the host bus adapter and the Storage Subsystem– Redundant paths eliminates host adapter, cable connection, and

channel adapters as single points of failures and increase availability– Improves performances by dynamically balancing workload across all

available paths

Application transparentClose integration with most UNIX and NT hostsSupported SCSI, Fibre Channel, and clustered environmentsGUI and CLI interface for management functions

Thanks for your attention. This ends our training on PowerPath Foundations.




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