Preventing fragmentation issues on virtualized Exchange 2010 mail servers
Microsoft did a good job driving down the database I/O requirements in Exchange
Server 2010. However, Exchange administrators still need to do their part to prevent
fragmentation from impacting I/O -- especially in virtualized Exchange Server environments.
There are several different methods to prevent virtualized mailbox server fragmentation because
fragmentation occurs at three different tiers: on the hypervisor storage level, on virtual hard
disk files and the mailbox database. Here’s how to manage defragmentation at each level.
The hypervisor storage level
The first tier where fragmentation occurs is the hypervisor storage level. This is especially true
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This was first published in July 2011
Exchange shops that use
Hyper-V,
but do not use a
storage area
network (SAN). In a SAN environment, each virtual hard drive file is traditionally placed on a
dedicated
logical unit
number (LUN). Smaller Exchange shops that do not use SANs often place multiple virtual hard
disk files onto a single hardware array.
This is problematic for two reasons. First, all of the virtual hard disk files that share the
array compete for disk I/O. You must use hardware that can provide sufficient I/O to keep up with
the demand to alleviate this problem.
Placing multiple virtual hard disk files onto a single hardware array can also be problematic
because Hyper-V creates dynamically expanding virtual hard disk files by default. These files
expand as data is added to the virtual hard disk. If multiple dynamically expanding virtual hard
disk files reside on a common storage array, then the virtual hard disk files can become fragmented
at the storage level.
If you're currently using Hyper-V, I strongly recommend only creating fixed length virtual hard
disk files. If you are using
VMware, this is a non-issue because VMware
creates fixed length virtual hard disks by default. Using fixed length virtual hard disk files
prevent storage level fragmentation from occurring.
Virtual hard disk file fragmentation
Fragmentation can also occur within a virtual hard disk (VHD) file. This is the same type of disk
fragmentation that occurs when Exchange Server is installed on a physical server; the only
difference is that the fragmentation occurs within a virtual hard disk rather than on a physical
hard disk.
The best strategy for minimizing this type of fragmentation is to configure your Exchange
mailbox server to use multiple virtual hard disks. For example, you could use one virtual hard disk
for the Windows OS, another for the pagefile, a third virtual
hard disk for the mailbox database and a fourth for the transaction logs. (This technique will not
entirely prevent fragmentation, but will certainly help to minimize it).
Mailbox database defragmentation
The third tier fragmentation can occur at is within the mailbox database. As database pages are
created and deleted, the database becomes fragmented. By default, Exchange
Server 2010 runs a nightly maintenance cycle that performs various housekeeping chores. The
most important chore is database defragmentation. This defragmentation does not remove empty space
or shrink the database, but it does reorganize the database pages to make read and write operations
more efficient.
The important thing to keep in mind is that the nightly maintenance cycle is extremely I/O
intensive. Microsoft automatically schedules maintenance to occur late at night to minimize the
performance impact caused by the defragmentation process.
For years, I've advised my clients and readers to modify the Exchange Server database
maintenance schedule so that it doesn't overlap with the nightly backup. Backing up a database
while simultaneously trying to defragment it can severely impact the server's performance.
Nightly backups are becoming less common as more and more Exchange shops are using continuous
backup solutions. Even if you fall into that category, it doesn't hurt to take a look at your
nightly maintenance schedule in your virtual environment.
If multiple virtual machines share a physical storage array, then it's possible that the nightly
database defragmentation process could be impacted by I/O-intensive tasks running on other virtual
servers. This includes virtual servers that are running applications other than Exchange Server
2010.
Ideally, you should avoid sharing physical storage devices among multiple virtual machines.
However, budgetary reasons often make this option unavoidable. In these situations, you should
determine exactly when each virtual machine produces the highest I/O and then schedule the Exchange
Server maintenance cycle to run during non-peak times. This will not only improve performance, but
will also reduce the chance that a storage bottleneck prevents Exchange from completing its
maintenance tasks within the allotted time.
In addition to the techniques that I have described above, there are also third-party products
specifically designed to combat fragmentation in virtual data centers. Two such products include PerfectDisk and Diskeeper.
ABOUT THE AUTHOR:
Brien Posey is an eight-time Microsoft MVP with two decades of IT experience. Before
becoming a freelance technical writer, Brien worked as a CIO for a national chain of hospitals and
healthcare facilities. He has also served as a network administrator for some of the nation’s
largest insurance companies and for the Department of Defense at Fort Knox.
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