Help Me Choose: Memory

• PowerEdge 13th-generation servers are very modular by nature, and you can now choose the exact quantity and type of DIMMs for your system.
• To configure your system’s memory for optimal performance, keep these best practices in mind:

1. Populate all memory channels evenly with the same rank, size, and speed of DIMMs. For situations that demand mixing DIMM sizes, all memory should be chosen based on identical rank structure, and all memory channels should be populated with an identical mix of DIMM types.
2. Choose the appropriate processor to support the expected memory frequency. For example, if your workload requires a memory speed of 2400MT/s, select a processor model that supports this speed.
3. For most system configurations, 1 DIMM per channel (DPC) and 2 DPC memory configurations perform better than 3 DPC because that memory frequency will be reduced when all slots in a given memory channel are populated.
4. Choose the Optimizer memory operating mode, which is part of the system profile, instead of Advanced ECC due to the significant performance penalties implicit in running the memory in this RAS mode. 
5. Choose LRDIMMs when the greatest capacity and performance of memory is desired. Choose RDIMMs for a balanced approach to memory capacity, performance and cost. Choose UDIMMs when using less power is a premium over capacity.
Memory Configuration Type
Reduce data loss and keep your system online and operational with reliability, availability and serviceability (RAS) features for Dell servers and storage, including:

High-reliability profile, which helps increase memory reliability through a selectable profile that adjusts parameters to reduce faults associated with speed, temperature, voltage and refresh rates. High reliability profile has a minor impact on power consumption and performance.

Memory mirroring has two adjacent memory channels configured to write the same data to each channel. If one memory channel fails or encounters an error, the other channel continues to transmit data. It’s an excellent safeguard for systems requiring uninterrupted operation, though it cuts memory capacity in half, doubles the cost per gigabyte, and can increase power consumption.

Memory sparing can reduce downtime from correctable errors by allocating one rank (64-bit wide data area on a DIMM) per channel as a spare. If a correctable error occurs in a rank or channel, it’s moved to the spare rank while the OS is running. This prevents the error from causing a failure. Memory sparing reduces memory capacity by one rank per channel and increases the cost per gigabyte.

DIMM Type
Choose from two different dual in-line memory module types or Dimm types that can meet your needs depending on how your prioritize R.A.S. (reliability, availability, and serviceability) and power consumption.

Dell is supporting three DIMM types on the thirteenth -generation servers:

  • RDIMM: Registered DIMM – Provides for higher capacity options and advanced RAS features.
  • LRDIMM: Load Reduced DIMM – Provides maximum capacity but higher power consumption.
  • UDIMM: Unbuffered DIMM – Provides low latency and low density.

RDIMM
, or registered memory, is the most commonly used DIMM type, and offers the best mix of frequency, capacity, and rank structure choices. It provides high signal integrity — performing parity checking to detect improper addresses or commands — and increased performance for heavy workloads.


RDIMMs
(Single Rank and Dual Rank)

  • Maximum frequency of 2400 MT/s
  • Maximum frequency using 3 DIMMs per channel of 1866 MT/s
  • Maximum capacity of 32GB per DIMM
  • Maximum system capacity of 768 GB


LRDIMM
, or load-reduced memory, uses a buffer to reduce memory loading to a single load on all DDR signals, allowing for greater density.

LRDIMMs

  • Maximum frequency of 2400MT/s
  • Maximum frequency using 3 DIMMs per channel of 2133MT/s
  • Maximum capacity of 128GB per DIMM
  • Maximum system capacity of 3TB

UDIMM,
is an unbuffered, low-density, low-latency DIMM that doesn’t include a register or a buffer chip. UDIMMs are generally used in applications requiring the lowest possible latency.

UDIMMs (Single Rank and Dual Rank)

  • Maximum frequency of 2400MT/s
  • Maximum capacity of 16GB per DIMM
  • Maximum system capacity of 64GB

DIMM Speed

There is only one speed option for DDR4
  • 2400MT/s – DIMMS will run at 1.2 volt

RAS Feature

RAS feature options offered on the thirteenth generation servers:
  • Memory Optimized – Baseline RAS features for highest performance optimization
  • Advanced ECC - RAS Feature that extends multi-bit single device error correction to x8 DRAMs
  • Memory Sparing- RAS feature that allocates one rank per channel as a spare to reduce the likelihood of correctable errors from becoming uncorrectable errors but available memory capacity is reduced
  • Memory Mirroring – RAS Feature with paired memory provides data and transmission fault-tolerance but available memory capacity is reduced by 50%
  • Dell Fault Resilient Memory (FRM) - This Dell-patented technology enables PowerEdge 13th generation server customers supporting VMware vSphere 5.5 to maximize available server memory while increasing protection against memory faults for the hypervisor and mission critical applications.
DIMM Speed and Frequency

The memory offerings for Dell PowerEdge 13th-generation servers are based on the double-data rate type 4 (DDR4) which operates at 1.2 volts.

PowerEdge memory speeds by type and loading

 DIMM Type  DIMM RankingCapacity DIMM Rated Voltage, Speed 13th Generation Memory (EP, 2/3
DPC platforms)
DPC means DIMMs per channel.
 1 DPC2 DPC 3 DPC

 RDIMM

1R/2R 

 4, 8, 16 and 32GB

DDR4 (1.2V), 2133 

2133

 2133

 1866

 LRDIMM

4R 

32GB, 64GB 

DDR4 (1.2V), 2133 

2133 

 2133

 1866

 UDIMM

1R/2R 

4, 8 and 16GB 

DDR4 (1.2V), 2133

2133 

2133 

 N/A

RDIMM 

1R/2R 

 4, 8, 16 and 32GB 

DDR4 (1.2V), 2400 

2400 

2400

 1866

 LRDIMM

4R 

64GB 

DDR4 (1.2V), 2400 

2400 

 2400

 2133

 LRDIMM

8R 

128GB 

 DDR4 (1.2V), 2400

2400

2400 

 2133

 UDIMM

 1R/2R

 4, 8 and 16GB

DDR4 (1.2V), 2400 

2400

 2400

 N/A



DIMM Comparison Table

The table below lists the comparison between different DIMM types.

 

     R DIMM

LR DIMM

 U DIMM

Registered/Buffered

  Address only

Address and data

 N/A

Frequency/Speed

2133 MT/s & 2400 MT/s

2133 MT/s & 2400 MT/s

 2133 MT/s & 2400 MT/s

Ranks Supported

1 or 2

4 only

 1 or 2

Capacity per DIMM

4, 8 , 16 GB and 32 GB

32GB (2133MT/s only) and 64GB

 4, 8 and 16GB

Maximum DIMM per Channel

3

3

 2

Dynamic RAM (DRAM) Technology

x4 and x8

x4

 x8

Temperature Sensor

yes

yes

yes 

Error-Correction Code (ECC)

yes

yes

yes 

Single Device Data Correction (SDDC)

yes
 (x8 requires Advanced ECC mode)

yes

yes 

Advanced ECC

yes

yes

 yes

Address Parity

yes

yes

yes