Storage Technology

Storage Technology

Exchange 2007

Storage Technology

Applies to: Exchange Server 2007, Exchange Server 2007 SP1 Topic Last Modified: 2008-03-20

The key aspects to choosing storage technology include reliability, capacity, performance, complexity, manageability, and cost. Microsoft Exchange Server 2007 enables a wider range of options for selecting storage technologies, such as serial ATA (SATA), serial attached SCSI (SAS), Internet SCSI (iSCSI), and Fibre Channel. This topic discusses each of these technologies in relation to Exchange 2007. In addition, information is provided about achieving redundancy for your storage design via types of redundant array of independent disks (RAID).

Unlike previous versions of Exchange Server, network-attached storage is not supported in Exchange 2007. The only network-based storage transport supported for Exchange 2007 is iSCSI.

Regardless of the solution you choose, all storage solutions used with Exchange 2007 must be listed on the Windows Server Catalog of Tested Products. In addition, single copy cluster (SCC) solutions must have the entire solution listed in the Cluster Solutions category of the Windows Server Catalog of Tested Products, and geographically dispersed SCC solutions must have the entire solution listed in the Geographically Dispersed Cluster Solutions category of the Windows Server Catalog of Tested Products.

Serial ATA

SATA is a new serial interface for Advanced Technology Attachment (ATA) and integrated device electronics (IDE) drives that are typically found in desktop computers. SATA drives are generally slower than small computer system interface (SCSI) and Fibre Channel disks, but they do come in large sizes. When considering SATA disks, we suggest that you check the manufacturer's recommendations about rotational vibration and heat tolerances. Some SATA disks have not been designed for disk arrays, and when you have too many disks close together, the resulting heat and vibrations cause disk errors and performance degradation. Also, you should make sure that the controller that will be used is a write caching array controller because that behavior will improve transactional throughput per spindle.

Serial Attached SCSI

SAS storage uses enterprise-class, high-performance hard disks. The throughput on many SAS arrays far surpasses both SATA and traditional SCSI (up to 3 gigabits per second) and may help to meet your service level agreement (SLA) for maintenance or backup (streaming performance). Many SAS arrays can be directly attached to the server, and the cabling is simple. Smaller form factor SAS disks come in smaller capacities, yet they are extremely fast, and are ideal for Exchange Server deployments where the fastest speeds are necessary with smaller mailboxes. It is important to balance the disk speed with the input/output (I/O) requirements. In many large mailbox deployments, 10,000 RPM SAS disks may be fast enough when you balance the capacity and I/O needs.

Internet SCSI

iSCSI is the only network-based storage that is supported by Exchange 2007. Although iSCSI connects a server to storage over Ethernet, it is important to treat it as your storage connection and completely isolate your iSCSI storage network from all other network traffic. If available, options such as flow control, quality of service (QoS), and jumbo frames can further increase performance. The Microsoft iSCSI Initiator 2.0 supports Multipath I/O (MPIO). In Microsoft test labs, over 250 megabytes (MB) per second have been pushed over three network cards, proving iSCSI as a capable storage transport for scenarios where high throughput is required.

If you choose iSCSI storage technology, it is very important that you configure the iSCSI initiator so that connected drives automatically reconnect after the server has been restarted. This is done by configuring the iSCSI initiator for persistent logon and with persistent volumes. If the iSCSI drives are not persisted after a restart, Exchange Server loses access to the drives.

Configuring persistence is particularly critical when using iSCSI with cluster continuous replication (CCR) and standby continuous replication (SCR). In addition, when using CCR or SCR, we strongly recommend that you make the Server service dependent on the Microsoft iSCSI Initiator service on the continuous replication source. (In the case of CCR, this should be done on both nodes, because the designations of active and passive changes during the life of the cluster.) This ensures that the disk volumes are present and that the file shares needed for continuous replication are properly created.

You can use the iSCSI command line interface (iSCSICLI) tool to configure a persistent logon target or the iSCSI Initiator Control Panel tool to make volumes persistent. You can also use the iSCSICLI command to bind persistent volumes or the iSCSI Initiator Control Panel tool to permit the iSCSI service to configure the list of persistent volumes.

For information about the iSCSICLI tool, see the Microsoft iSCSI Software Initiator 2.x User's Guide. For detailed steps about how to configure iSCSI targets and volumes for persistent logon and volumes, and to configure the Server service so that it depends on the Microsoft iSCSI Initiator service, see Microsoft Knowledge Base article 870964, File shares on iSCSI devices may not be re-created when you restart the computer.

Fibre Channel

Fibre Channel is a network technology often using fiber optic cables in storage area networks (SANs). It is a gigabit speed network that is high performing and excellent for storage consolidation and management. If you are using Fibre Channel storage, we recommend that you check with your storage vendor for optimal configuration settings because each storage vendor has recommendations that should be followed for the Queue Depth, Queue Target, or Execution Throttle settings.

RAID Selection

Adding redundancy to your storage design is critical to achieving high availability. RAID storage behind a battery-backed controller is highly recommended for all Exchange servers. There are many RAID types, and many proprietary modifications to the known RAID types. However, the four most common types used in server environments are RAID-1/0, RAID-5, RAID-6, and RAID-DP.

The following table compares RAID-1/0, RAID-5, and RAID-6 solutions based on speed, space utilization, and performance during rebuilds and failures.

Comparison of RAID solutions

RAID type	Speed	Capacity utilization	Rebuild performance	Disk failure performance	Transactional I/O performance
RAID-1/0	Best	Poor	Best	Best	Best
RAID-5	Good	Best	Poor	Poor	Poor
RAID-6*	Poor	Good	Poor	Poor	Poor

Note:
*The performance for RAID-6 varies depending on disk layout, storage controller, and storage configuration. Talk to your storage vendor for detailed performance information for RAID-6 solutions.

RAID-1/0

RAID-1/0 is where data is striped (RAID-0) across mirrored (RAID-1) sets. RAID-0-1 is not the same as RAID-1/0, and we do not recommend RAID-0-1 for Exchange data. Transactional performance with RAID-1/0 is very good because either disk in the mirror can respond to read requests. No parity information needs to be calculated, so disk writes are efficiently handled. Each disk in the mirrored set must perform the same write.

When a disk fails in a RAID-1/0 array, write performance is not affected because there is still a member of the mirror that can accept writes. Reads are moderately affected because now only one physical disk can respond to read requests. When the failed disk is replaced, the mirror is again established, and the data must be copied or rebuilt.

RAID-5

RAID-5 involves calculating parity that can be used with surviving member data to re-create the data on a failed disk. Writing to a RAID-5 array causes up to four I/Os for each I/O to be written, and the parity calculation can consume controller or server resources. Transactional performance with RAID-5 can still be good, particularly when using a storage controller to calculate the parity.

When a disk fails in a RAID-5 array, the array is in a degraded state, and performance is less and latencies are higher. This situation occurs because most arrays spread the parity information equally across all disks in the array, and it can be combined with surviving data blocks to reconstruct data in real time. Both reads and writes must access multiple physical disks to reconstruct data on a lost disk, thereby increasing latency and reducing performance on a RAID-5 array during a failure. When the failed disk is replaced, the parity and surviving blocks are used to reconstruct the lost data, which is a lengthy process that can take hours or days. If a second member of the RAID-5 array fails during the Interim Data Recovery Mode or rebuild, the array is lost. Because of this vulnerability, RAID-6 was created.

RAID-6

RAID-6 adds an additional parity block and provides approximately double the data protection over RAID-5, but at a cost of even lower write performance. As physical disks grow larger, and consequently RAID rebuild times grow longer, in some cases RAID-6 is necessary to prevent logical unit number (LUN) failure if an uncorrectable error occurs during the rebuild, or if a second disk in the array group fails during rebuild. Due to disk capacity, some vendors support RAID-6 instead of RAID-5.

Note:
For more information about the Storage Network Industry Association definition of RAID-6, see SNIA Dictionary Links. The third-party Web site information in this topic is provided to help you find the technical information you need. The URLs are subject to change without notice.

RAID-DP

RAID-DP from NetApp is a proprietary implementation of RAID double parity for data protection. RAID-DP falls within the Storage Network Industry Association definition of RAID-6. RAID-DP is also a trademark of NetApp.

Unlike traditional RAID-6, RAID-DP utilizes diagonal parity using two dedicated parity disks in the RAID group. RAID-DP is also similar to other RAID-6 implementations in terms of the reliability metrics and its ability to survive the loss of any two disks; however, a third disk failure will result in data loss. Whereas current RAID-6 implementations incur an I/O performance penalty as a result of introducing an additional parity block, RAID-DP is optimized in terms of reducing read I/Os due to the way the NetApp controller handles parity write operations. Unlike other storage controllers that write changes to the original location, the NetApp controller always writes data to new blocks, thus making random writes appear to be written sequentially. It is important to follow NetApp best practices for sizing the array to ensure a consistent level of performance for Exchange implementations.

Note:
For more information about RAID-DP, see "RAID-DP: Network Appliance Implementation of RAID Double Parity for Data Protection" at http://www.netapp.com/library/tr/3298.pdf and "Using NETAPP RAID-DP in Exchange Server 2007 Storage Designs" at http://www.netapp.com/library/tr/3574.pdf, or contact NetApp directly. The third-party Web site information in this topic is provided to help you find the technical information you need. The URLs are subject to change without notice.

Note:

For more information about RAID-DP, see "RAID-DP: Network Appliance Implementation of RAID Double Parity for Data Protection" at http://www.netapp.com/library/tr/3298.pdf and "Using NETAPP RAID-DP in Exchange Server 2007 Storage Designs" at http://www.netapp.com/library/tr/3574.pdf, or contact NetApp directly. The third-party Web site information in this topic is provided to help you find the technical information you need. The URLs are subject to change without notice.

Selecting a RAID Type

Selecting a RAID type is a balance of capacity, transactional I/O, and failure or rebuild performance characteristics. For example, mailbox size has a large impact on capacity, while smaller form factor disks impact performance. The type of RAID to select also depends on the data being stored and the controller being used. Transaction logs are the most important data set, and good write latency is critical for server performance. When using a storage controller that is RAID agnostic, transaction logs should be placed on RAID-1 or RAID-1/0 arrays with battery-backed write cache. For more information about the importance of quick, low-latency storage for the transaction logs, see Optimizing Storage for Exchange Server 2003. Likewise, when using a storage controller that is RAID agnostic, RAID-1/0 is the ideal configuration for databases, and it works well with large capacity disks.

In Exchange Server 2003, RAID-5 provided the best capacity efficiency, although its poorer performance seldom allowed the extra space to be used. As a result, in many Exchange 2003 deployments, more physical disks were required to meet the transactional performance requirements of RAID-5 than with RAID-1/0.

With Exchange 2007, the shift of increasing database writes as a percentage of database I/O causes RAID-5 LUNs to perform worse than in Exchange 2003. However, when following the recommendations to achieve a transactional I/O reduction, RAID-5 may be a good solution. RAID-5 is useful for using high speed, smaller capacity disks. In large mailbox solutions, RAID-5 may be able to provide more transactional performance than you need, to meet the capacity requirements with less physical disks than RAID-1/0.

For both RAID-5 and RAID-6, rebuild performance can have a significant effect on storage throughput. Depending upon the storage array and configuration, this effect could cut storage throughput in half. Scheduling rebuilds outside of production hours can offset this performance drop, but doing so sacrifices reliability. In a CCR environment, you can prevent the throughput reduction affecting users by moving the Mailbox server to the passive node, thereby making it the active node. If neither option is available, additional I/O throughput should be designed into the architecture to accommodate RAID-5 or RAID-6 rebuild conditions during production hours. This additional I/O throughput can be up to twice the non-failed state I/O requirements.

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