Establishing Service Level Agreement Requirements

Gilt für: Exchange Server 2007 SP1, Exchange Server 2007

Letztes Änderungsdatum des Themas: 2006-11-12

One of the first steps in planning your backup and restore strategy is to review the service level agreement (SLA) that you have established for your organization. After you establish an SLA, you can determine the Microsoft Exchange Server 2007 deployment and server configurations that are best suited for that agreement.

The following are the key considerations for high availability as they relate to disaster recovery:

• Allowed downtime   Consider the maximum allowed downtime that is acceptable for your organization based on your organization's definition of Exchange service availability. Depending on your organization's definition of downtime, you may be able to meet your organization's SLA by using a messaging dial tone recovery strategy. A messaging dial tone recovery strategy involves providing your users with a temporary mailbox so that they can send and receive e-mail messages immediately after a disaster. This strategy quickly restores e-mail service in advance of recovering historical mailbox data. Typically, recovery will be completed by eventually merging historical and temporary mailbox data.
• Allowed recovery time   Consider the maximum time allowed for each type of disaster recovery operation. For example, you should specify the approximate period of time it takes to recover a mailbox, a single database, or an entire server that is running Exchange 2007.
• Data loss tolerance   Consider the tolerance your organization has for either the temporary or permanent loss of Exchange data. For example, your organization may be able to tolerate the temporary loss of mailbox data since the previous backup for a period of 24 hours, as long as users can send and receive messages within a 4-hour time period. In other cases, you may want to consider a stricter requirement, such as requiring that all Exchange data up to the point of failure be restored within 4 hours.

After considering the impact of downtime on your organization and deciding on a level of uptime that you want to achieve in your messaging environment, you are ready to establish an SLA. SLA requirements determine how components such as storage, clustering, and backup and recovery factor into your organization.

When assessing SLAs, you should start by identifying the hours of regular operation and the expectations regarding planned downtime. You should then determine your company's expectations regarding availability, performance, and recoverability, including message delivery time, percentage of server uptime, amount of storage required, and time to recover an Exchange database.

Additionally, you should identify the estimated cost of unplanned downtime so that you can appropriate the proper amount of fault tolerance into your messaging system.

Features in Exchange 2007 and Microsoft Windows Server 2003 may affect how you design your organization to meet SLAs. For example, local continuous replication (LCR), cluster continuous replication (CCR), single copy clusters (SCC), the Volume Shadow Copy Service (VSS), recovery storage groups, database portability, and dial tone portability features could allow you to challenge the limits that were previously imposed by your SLAs.

Table 1 lists some of the categories and specific elements that you may want to include in your SLAs.

Table 1   Categories and elements in a typical enterprise-level SLA

Including a variety of performance measures in your SLAs helps make sure that you are meeting the specific performance requirements of your users. For example, if there is high latency or low available bandwidth between clients and mailbox servers, users would view the performance level differently from system administrators. Specifically, users would consider the performance level to be poor, although system administrators would consider the performance to be acceptable. Therefore, make sure that you monitor disk I/O latency levels.

Establishing Service Level Agreements with Your Vendors

Many businesses that place importance on high availability solutions use the services of third-party vendors to achieve their high availability goals. In these cases, achieving a highly available messaging system requires services from outside hardware and software vendors. Unresponsive vendors and poorly trained vendor staff can reduce the availability of the messaging system.

Make sure that you negotiate an SLA with each of your major vendors. Establishing SLAs with your vendors helps guarantee that your messaging system performs to specifications, supports required growth, and is available to a specific standard. The absence of an SLA can significantly increase the length of time that the messaging system is unavailable.

In addition to establishing an SLA with your major vendors, you should also periodically test escalation procedures by conducting support-request drills. To confirm that you have the most recent contact information, make sure that you also test pagers and telephone trees.