Overview Robert's Rules of Exchange is a series of blog posts in which we take a fictitious company, describe their existing Exchange implementation, and then walk you through the de...
Updated Jul 01, 2019
Version 2.0
Blog Post
@Daniel Eason - Good questions, thanks! What you cannot do with direct attached is "hardware VSS". Companies like EMC and NetAPP have some interesting hardware VSS solutions for backing up Exchange data. With a DAS solution, you have to look at "software VSS" solutions like Microsoft's DPM or others (IBM's Tivoli has a software VSS that some of my customers use, and I believe there are others out there). This is certainly something to consider when you design your Exchange infrastructure.
As for hot growth or dynamically adding disks or volumes to your Exchange systems, that is certainly something that SANs allow that DAS makes more difficult. The question there still comes down to a few things.
First, if you engineer DAS/JBOD for growth, looking at a vision of how much data the users will store over time, you can predict how much data space you will need. By doing this, you can purchase enough JBOD storage to house the space needed by your users over the 3 or 5 year life of the hardware systems.
Second, you can do something called oversubscription of your disks. This is where you actually purchase less disks than you provision in your mailbox quotas. If you give 1000 users 10GB mailbox limits, you need 10,000GB of space, right? But if you only purchase 5,000GB of disk, that is oversubscibing your disks. Then later, if you run out of space, you can then purchase more storage. SANs allow for this.
BUT... You must take into account that the users will generate the same number of IOPS (basically) with a 5GB mailbox as they would with a 10GB mailbox. So if you purchase enough spindles to give them space for 5GB, you will then later have to purchase the same performance level of spindles so that you can extend the disks/volumes available to Exchange while giving the same IO performance.
Microsoft's recommendation is to purchase the hardware (servers, processors, memory, disks, everything) such that you are making full use of the hardware as deployed, planning for worst case in your growth scenarios and worst case in your failures (both). Then, if you need to grow your system, you plan for adding more servers to the DAGs with disks on those servers.
This is the harder part of planning Exchange. Most of us don't have a "crystal ball" and training at Hogwarts such that we know how to tell the future. But, quite frankly, most of my customers can project growth, and be fairly accurate. My customers typically don't dynamically grow their Exchange storage over the years. They set limits and stick with them. Of course, YMMV. ;*)
As for hot growth or dynamically adding disks or volumes to your Exchange systems, that is certainly something that SANs allow that DAS makes more difficult. The question there still comes down to a few things.
First, if you engineer DAS/JBOD for growth, looking at a vision of how much data the users will store over time, you can predict how much data space you will need. By doing this, you can purchase enough JBOD storage to house the space needed by your users over the 3 or 5 year life of the hardware systems.
Second, you can do something called oversubscription of your disks. This is where you actually purchase less disks than you provision in your mailbox quotas. If you give 1000 users 10GB mailbox limits, you need 10,000GB of space, right? But if you only purchase 5,000GB of disk, that is oversubscibing your disks. Then later, if you run out of space, you can then purchase more storage. SANs allow for this.
BUT... You must take into account that the users will generate the same number of IOPS (basically) with a 5GB mailbox as they would with a 10GB mailbox. So if you purchase enough spindles to give them space for 5GB, you will then later have to purchase the same performance level of spindles so that you can extend the disks/volumes available to Exchange while giving the same IO performance.
Microsoft's recommendation is to purchase the hardware (servers, processors, memory, disks, everything) such that you are making full use of the hardware as deployed, planning for worst case in your growth scenarios and worst case in your failures (both). Then, if you need to grow your system, you plan for adding more servers to the DAGs with disks on those servers.
This is the harder part of planning Exchange. Most of us don't have a "crystal ball" and training at Hogwarts such that we know how to tell the future. But, quite frankly, most of my customers can project growth, and be fairly accurate. My customers typically don't dynamically grow their Exchange storage over the years. They set limits and stick with them. Of course, YMMV. ;*)