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Microsoft Exchange Server 2010 Requirements : Getting the Right Server Hardware (part 3) - Disk Requirements

10/18/2014 3:57:18 AM
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5. Disk Requirements

When calculating disk requirements for some applications, it is easy to decide that a single 500 GB hard disk will solve your storage needs. You might be tempted to think the same thing about Exchange Server.

With earlier versions of Exchange, getting the disk requirements sized correctly could be a bit tricky. That is not to say that doing so cannot still be tricky with Exchange Server 2010. This is because sizing a disk is not just a matter of figuring out how much storage you need. Physical storage requirements are a big part of the sizing, of course, because if you don't get large enough disks to support your users, you will be going back to the boss for more money to buy more disks.

But, asking the boss to buy more physical disk drives because the user's mailboxes are full is at least something tangible you can ask for. The other side of the sizing requirement is ensuring that the disk I/O capacity will keep up with the database engine. The more users using the Exchange Server, the greater the disk I/O capacity required by the disk subsystem. Try explaining to your boss that the disks have plenty of storage available but can't keep up with the database load.

The disk subsystem that you choose has to be able to support not only the amount of storage required but also the I/O load that the users will place on the disk subsystem. Therefore, understanding the I/O profile as well as the amount of storage required is important.

5.1. Improved Caching and Reduced I/O Profiles

By and large, Client Access and Hub Transport servers require far less disk I/O capacity than Mailbox servers, though Hub Transport servers in very large messaging environments may need more I/O than most organizations. The information in this section applies to servers that are hosting the Mailbox server role.

If you are coming from the Exchange Server 2000/2003 world, you already know that even on a server with only a few hundred mailboxes, Exchange Server 2000/2003 quickly reaches the maximum amount of RAM available for caching (1.2 GB maximum). As more and more users vie for the same physical memory for caching, Exchange Server quickly becomes constrained by the amount of I/O operations that the Exchange server's disk subsystem can support.

Hundreds of pages of material have been written on the concept of optimizing Exchange Server for maximizing performance by improving I/O performance with Exchange — and we certainly can't do the concept justice in just a few paragraphs—but understanding the basic input/output per second (IOPS) requirements of users is helpful. Microsoft and hardware vendors have done much research on I/O requirements based on the mailbox size and the average load that each user places on the server.

Remember the user profile table shown previously in Table 1? Well, Table 6 takes that and includes the estimated IOPS given a user type and an estimated mailbox size for Exchange 2003. We are including this information because we want you to see the database performance improvements since Exchange Server 2003. IOPS requirements climb as the number of messages sent and received increases and as the mailbox size increases.
Table 6. User Type, Database IOPS, Messages Sent and Received, and Mailbox Size Estimates for Exchange 2003
User TypeDatabase Volume IOPSMessages Sent/Received per DayMailbox Size
Light.520 sent/50 received50 MB
Average.7530 sent/75 received100 MB
Heavy1.040 sent/100 received200 MB
Large1.560 sent/150 received500 MB


For an Exchange 2003 server that is supporting 3,000 heavy mailbox users, the disk subsystem would have to support at least 3,000 IOPS. A typical SCSI or SAS disk drive supports between 100 and 150 IOPS (depending on the disk drive model). To meet this requirement, the disk subsystem may require more disks (from an I/O capacity perspective) than are required from a disk space perspective; thus, the disk subsystem may have far more disk space than is actually necessary to support the IOPS profile. Failure to plan for sufficient IOPS capacity on the disk subsystem will significantly hurt performance.

When Exchange Server 2007 entered the market, the 64-bit architectural improvements allowed the operating system and Exchange Server 2007 to access more physical memory. With additional physical memory available for caching, disk I/O is significantly reduced. Microsoft estimates that I/O requirements are reduced by approximately 70 percent provided the Exchange 2007 server has the recommended amount of RAM. Table 7 shows the estimated IOPS requirements for Exchange 2007 Mailbox servers. Please keep in mind that these are estimates and may change over time. These numbers are also calculated when the Mailbox server is configured with more than the recommended amount of RAM.

With this significant improvement in caching Exchange data, the Extensible Storage Engine (ESE) database engine needs to read and write from the disk less frequently and thus reduces the IOPS requirements. When the IOPS requirements are reduced, fewer disks are required to support the I/O load. Notice an Exchange Server 2007 "heavy" user requires only 0.32 IOPS as opposed to an Exchange 2003 "heavy" user that requires 1.0 IOPS.

The Exchange database team has been hard at work further improving the I/O performance of Exchange Server 2010 Mailbox. One of the key factors that the database team focused on with Exchange Server 2010 is to further improve the I/O performance so that most types of affordable disk drive can be used (such as SATA, SAS, or SCSI). They have done this by further optimizing the use of cache memory, increasing database page sizes, changing the database schema, and optimizing how the database arranges data to be written to the disk.

The resulting improvements to the Exchange Server 2010 database engine further reduce the I/O requirements for the standard usage profiles. Table 8 shows the disk I/O recommendations based on usage profiles for Exchange Server 2010. Note that the estimates in Table 8 are based on the release-to-manufacturing version of Exchange Server 2010 and Microsoft may refine these further in the future.

Table 7. User Type, Database Volume IOPS, and Messages Sent and Received Per Day for Exchange 2007
User TypeDatabase Volume IOPSMessages Sent/Received per Day
Light.115 sent/20 received
Average.1810 sent/40 received
Heavy.3220 sent/80 received
Very Heavy.4830 sent/120 received
Extra Heavy.6440 sent/160 received



Table 8. User Type, Database Volume IOPS, and Messages Sent and Received per Day For Exchange 2010
User TypeDatabase Volume IOPSMessages Sent/Received per Day
Light.105 sent/20 received
Average.1410 sent/40 received
Heavy.2020 sent/80 received
Large.2930 sent/120 received


The I/O requirements, of course, are just estimates, but they generally provide a pretty good guideline for the IOPS requirements for the disks that will host Exchange databases. The disks that will host the Exchange transaction logs will require approximately 10 to 20 percent of the IOPS requirements for their corresponding database.

Improvements in Disk I/O and How This Affects Storage Costs

In many environments with more than a few hundred mailboxes, the storage subsystem becomes the most expensive part of the Exchange infrastructure.

Company XYZ had a single Exchange 2003 server that supported approximately 1,500 users. Using Performance Monitor, they estimated that the average IOPS requirement was approximately 0.75 IOPS per second. The disk subsystem that held the database therefore had to support approximately 1,125 IOPS. To give themselves some room to grow and to accommodate unusual spurts in activity, the company used an estimated value of 1,500 IOPS.

Based on the architecture of the physical server they were using for their Exchange 2003 Mailbox server role, the company could not achieve this IOPS requirement using direct attached storage (DAS). Therefore, they had to use a fiber channel storage area network (SAN) to accommodate the Exchange data. The cost per gigabyte for the SAN storage was approximately $38.

During their planning for Exchange Server 2010, the company estimated that the typical user was somewhere between an average user and a heavy user. They further estimated that the IOPS requirement for each user would be approximately 0.20 IOPS per user, or a total of 300 IOPS. This represented a significant drop in the IOPS requirements from Exchange Server 2003. With their proposed server architecture, they could accommodate this IOPS requirement with DAS for a cost of approximately of $5 per gigabyte.

Granted that a SAN can often provide more features (scalability, snapshots, replication, and so forth) than just raw storage, but this company had to weigh the costs for those additional features against their relative value to the company. In this case, they chose to use DAS instead of the SAN and saved a considerable amount of money.


5.2. Mailbox Storage

Exchange servers holding the Mailbox server role consume the most disk space. Exchange system designers often fall short in their designs by not allowing sufficient disk space for mail storage, transaction logs, and extra disk space. Often the disk space is not partitioned correctly, either. Here are some important points to keep in mind when planning your disk space requirements:

  • Transaction log files should be on a separate set of physical disks (spindles) from their corresponding Exchange database files if you are only deploying a single database copy. RAID 1 or RAID 0+1 arrays provide better performance for transaction logs.

  • Allow for at least 7 to 10 days' worth of transaction logs to be stored for each database. The estimated amount of transaction logs will vary dramatically from one organization to another, but a good starting point is about 4 GB of transaction logs per day per 1,000 mailboxes. This is just one estimate of a specific usage profile, though, and your actual mileage may vary. Tools like the Exchange Storage Calculator can be used to assist in disk space requirements.

  • If you frequently move mailboxes from one mailbox database to another, take this into consideration. When a mailbox is moved in Exchange 2010, the mailbox's dumpster is moved with the mailbox.

  • Allow for whitespace estimates in the maximum size of each of your database files. (The whitespace is the empty space that is found in the database at any given time.) The size of the whitespace in the database can be approximated by the amount of mail sent and received by the users with mailboxes in that database. For example, if you have one hundred 2 GB mailboxes (a total of 200 GB) in a database where users send and receive an average of 10 MB of mail per day, the whitespace is approximately 1 GB (100 mailboxes × 10 MB per mailbox).

  • Factor in 5 to 10 percent additional disk space for the content index databases. You will have one content index database for each production database.

  • Allocate enough free space on the disk so that you can always make a backup copy of your largest database and still have some free disk space. A good way to calculate this is to take 110 percent of the largest database you will support because that also allows you to defragment the database using Eseutil if necessary.

  • Consider additional disk space for message tracking, message transport, RPC Client Access, HTTP protocol, POP3 protocol, and IMAP4 protocol log files if you have combined function servers.

  • Always have recovery in mind and make sure you have enough disk space to be able to restore a database to a recovery database.

Microsoft has a number of excellent guidelines for estimating disk space requirements and database sizing, including the Storage Calculator.


Let's move on to an example of a server that will support 1,000 mailboxes. We are estimating that we will provide the typical user with a Prohibit Send size warning of 500 MB and a Prohibit Send And Receive limit of 600 MB. In any organization of 1,000 users, you have to take into account that 10 percent will qualify as VIPs who will be allowed more mail storage than a typical user; in this case, let's allow 100 VIP users to have a Prohibit Send And Receive limit of 2GB.

These calculations result in 540 GB of mail storage requirements (600 MB × 900 mailboxes) for the first 900 users plus another 200 GB (2 GB × 100 mailboxes) for the VIP users. This results in a maximum amount of mail storage of 740 GB. However, this estimate does not include estimates for deleted items in a user's mailbox and deleted mailboxes, so we want to add an additional overhead factor of about 15 percent, or about 111 MB, plus an additional overhead factor of another 15 percent (another 111 MB) for database whitespace.

So at any given time, for these 1,000 mailboxes we can expect mail database storage (valid email content, deleted data, and empty database space) to consume approximately 962 GB, but because we like round numbers, we'll average that up to 1,000 GB, or 1TB.

In this example, let's say that we have decided the maximum database size we want to be able to back up or restore is 100 GB. This means that we need to split the users' mailboxes across 10 mailbox databases.

For the transaction logs, we estimate that we will generate approximately 5 GB of transaction logs per day. We should plan for enough disk space on the transaction log disk for at least 50 GB of available disk space.

Next, because full-text indexing is enabled by default, we should allow enough disk space for the full-text index files. In this case, we will estimate that the full-text index files will consume a maximum of about 10 percent of the total size of the mail data, or approximately 100 GB. If we combine the full-text index files on the same disk drive as the database files, we will need about 1.3 TB of disk space.

Anytime you are not sure how much disk space you should include, it is a good idea to plan for more rather than less. Although disk space is reasonably inexpensive, unless you have sophisticated storage systems, adding additional disk space can be time consuming and costly from the perspective of effort and downtime.

5.3. Planning for Mail Growth

Growth? You may be saying to yourself, "I just gave the typical user a maximum mailbox size of 600 MB and the VIPs a maximum size of 2 GB! How can my users possibly need more mailbox space?" Predicting the amount of growth you may need in the future is a difficult task. You may not be able to foresee new organizational requirements or that you might be influenced by future laws that require specific data retention periods.

In our experience, though, mailbox limits, regardless of how rigid you plan to be, are managed by exception and by need. In the preceding example, we calculated that we would need 1.3 TB of disk space for our 1,000 mailboxes. Would we partition or create a disk of exactly that size? Probably not.

Instead of carving out exactly the amount of disk space you anticipate needing, add a "fluff factor" to your calculations. We recommend adding approximately 20 to 25 percent additional capacity to the anticipated amount of storage you think you will require, but this is just a wild guess. In this example, though, we might anticipate using 1.3 TB of disk space if we added 25 percent to our expected requirements. Here are some factors that you may want to consider when deciding how much growth you should expect for your mailbox servers:

  • Average annual growth in the number of employees

  • Acquisitions, mergers, or consolidations that are planned for the foreseeable future

  • Addition of new mail-enabled applications such as Unified Messaging features or electronic forms routing

  • Government regulations that require some types of corporate records (including email) to be retained for a number of years

Conversely, potential events in your future could reduce the amount of mailbox storage you require. Many organizations are now including message archival and long-term retention systems in their messaging systems. These systems archive older content from a user's mailbox and move it to some type of external storage such as disk, storage area network (SAN), network-attached storage (NAS), optical, or tape storage.

5.4. Email Archiving and Mail Storage

Email has emerged as the predominant form of business communications. Sales, marketing, ordering, human resources, legal, financial and all other types of information are now disseminated via email.

An emerging trend in the email business is email archiving. As of this writing, more than 60 companies provide archiving solutions for email systems. Some of these companies provide in-house solutions whereas some are hosted solutions. There are just about as many reasons to implement an email archive system as there are archive vendors. Some of the reasons to implement email archiving include:

  • Reduces the size of mailbox databases and mailboxes (smaller databases and smaller mailboxes improve disaster recovery response times and improve performance)

  • Provides long-term retention of email data

  • Provides users with a searchable index of their historical email data

  • Allows for eDiscovery of email (message content, attachments, as well as email metadata) that often must be indexed for legal proceedings

  • Eliminates the use of Outlook personal folder (PST) files

Third-party archive systems are great for organizations that must retain much of the information in their mailboxes but want to move it to external storage. However, depending on the system, you don't want to archive everything older than five days, because that may prevent the user from accessing it via Outlook Web Access or mobile devices. Further, once the content is archived and no longer residing in the user's mailbox, it will no longer be accessible from a user's desktop search engine, such as the Google Desktop or the Windows Desktop search engine. So keeping a certain amount of content in the user's mailbox will always make sense.

Microsoft has introduced an email archive system for Exchange Server 2010. Microsoft's approach in this version is to establish an extra archive mailbox for each user who requires archiving. The email archive mailbox must reside on the same mailbox database as the user's mailbox. This approach does serve the goal of reducing the size of the user's primary mailbox, but it does not reduce the size of the database.

If you are planning to use the Exchange Server 2010 mailbox archive feature, you will need to take this into account and plan for additional storage as needed.

 
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