.Part 4 SAN Extension and Bridging HP SAN Design Reference Guide 785354-001

Exchange storage design
Important criteria for Exchange storage design include:
• Isolation of Exchange transaction logs from databases
• Selection of optimum RAID level for performance and fault tolerance
• Write-back caching for hardware RAID controller performance
Separate volumes for logs and databases
HP recommends separate volumes for Exchange transaction logs and databases, to ensure data
protection and efficiency. Transaction log access is mostly sequential writes does database access
is random read/write.
The Exchange server internal storage can hold the Exchange transaction logs while the HP Storage
Server with the HP iSCSI Feature Pack holds the Exchange databases.
The Exchange server internal storage and the iSCSI NAS Storage Server have comparable
transaction log performance. If the transaction logs are stored on the Exchange server, you must
recover them manually if the server fails. Regaining access to the log drives requires installing the
logs on a backup Exchange server.
NOTE: Store transaction logs on a RAID 1 mirror pair array volume, or, for additional disk space,
on four or more disks on a RAID 1+0 (striped mirror).
RAID level
For database volumes, the choice of RAID protection on the disk arrays is a trade-off between
maximum storage and performance. For the same number of disk spindles, RAID 5 provides data
protection and maximum storage, and RAID 1+0 provides the best performance but with less
storage.
Six spindles in a RAID 1+0 array provide greater performance than six spindles in a RAID 5 array
for a given number of Exchange mailboxes. RAID 1+0 is preferred for the database volume. Using
six 36 GB drives in a RAID 1+0 array provides ample storage and performance for 1,000 100
MB mailboxes.
Designing storage arrays with the large disk drives (146 GB or larger) requires caution. Although
a few large disks can provide the required amount of database storage, the reduced spindle count
will decrease I/O performance.
Write-back caching
Write-back caching increases performance for transaction logs on the server array and for databases
on the storage array. On array controllers with battery-backed write cache (such as the Smart
Array 5i Plus and later) the write-cache percentage should be set to 100%.
Recommendations
HP recommends that you:
• Place the Exchange log files and database files on separate RAID 1+0 RAID sets.
• Place the Exchange log files on Exchange server disks.
• Use hardware RAID controllers with write-back caching.
Supported load with Exchange
The performance required by the average email user determines the storage design. The average
load is multiplied by the number of users to find the storage requirement. Conversely, the capabilities
of an existing system can determine the maximum number of users.
To calculate the average I/O per user in an Exchange environment, the PERFMON object's
disk-transfers-per-second value is divided by the number of active connections. The storage capacity
HP iSCSI bridge products 363