I’m working on specs for some servers that will run MS SQL server. I have discovered some interesting things about memory.

Feature Details FAQ Glossary
System RAM, or memory, refers to the amount of data that can be stored on its journey from the hard drives to the processors, and vice versa. With more RAM, the processor will have to retrieve information from the hard drives less often, speeding the overall performance of the system. While some applications, such as email, require more RAM than others, all applications can benefit from increased memory. Dell servers feature DDR2 memory, which enables systems to improve performance and reduce power consumption.

Select the right amount of memory based on your application and number of users
4GB 8GB 16GB+
File sharing/print 50-200 users 200-500 users 500+ users
Messaging and Collaboration – Email 50-200 users 200-400 users 400+ users
Web Server/Caching 200+ users
Infrastructure/Systems Management 200+ users
Database 50-150 users 150-250 users 250+ users
Host Multiple Applications 50-150 users 150-250 users 250+ users
ERP, CRM, or other business applications 25-100 users 100-250 users 250+ users
Virtual Environments 4-8 virtualized servers >8 virtualized servers

What’s the difference between Memory Mirroring, and Memory RAID?

Memory Mirroring can be thought of as RAID1 for memory – where a pair of DIMMs is copied exactly by another pair of DIMMs. In the event that the first, or active, pair reaches a high error threshold, the secondary, or backup pair is activated. Memory RAID is like RAID5 for memory. It is not on a 1:1 ratio, and requires that all DIMMs be populated. The final pair of DIMMs serves as the backup pair, while all others are active. If any pair exceeds the error threshold, the backup pair activates.

When would I choose Single-Ranked DIMMs, and when Dual-Ranked DIMMs?

This is a matter of price/performance versus scalability. For example, on the PowerEdge 2850, if you selected 8GB of RAM on a system as Dual-Ranked DIMMs, they will address all the memory “ranks” available, thereby maximizing performance for that 8GB of RAM. This while populating only 4 out of 6 slots. However, if the system only had six slots, the 2GB Dual-Ranked DIMMs would consume four of those slots and would disable the two empty slots, essentially maxing out the RAM at 8GB. Meanwhile, 8GB on Single-Ranked DIMMs, which also would consume four slots, would leave the remaining two slots fully usable, allowing you to add more RAM later. However, only two-thirds of the ranks would be addressed, offering less overall performance than the Dual-Ranked 8GB of RAM. So if you think you might need to upgrade your RAM later, Single-Ranked DIMMs are probably the way to go. But if you’re looking for more bang for your buck, and you don’t plan on adding RAM later, Dual-Ranked DIMMs are a great option.