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Turning to 2U, 4U servers in virtualized data centers

As managers virtualize their data centers, they often turn to larger 2U and 4U servers for better performance. But these form factors can also pose challenges for administrators.

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One of the biggest trends in corporate data centers is server virtualization. Although server virtualization is designed to reduce physical server sprawl, host servers tend to require powerful hardware if they are to host more than a handful of virtual machines (VMs). As such, many organizations have turned to 2U and 4U servers, because these server form factors can deliver the performance required of a virtual data center. But while these larger server form factors can offer improved virtualization performance and room for expansion, they also pose some challenges, such as drive support, storage, power supply and management capabilities.

This article outlines the most important criteria that should be examined when purchasing a 2U or a 4U server. We’ll also examine the servers’ internal and external hardware, as well as their management capabilities.


Internal 2U, 4U server characteristics

Form factor
Rackmount servers use a unit of measurement known as a U (or sometimes RU). A U represents one rack unit. As such, a 1U server is 1.75 inches high and either 19 or 23 inches wide (although the server’s front panel may be slightly larger).

A 2U server is a server that is designed to take up two slots on a rack and is therefore two rack units in size. A 2U server is more than 3.5 inches thick (twice the thickness of a 1U server), because server racks allow for gaps between servers. Hence a 2U server occupies the space that would normally be consumed by two 1U servers and the gap between them. Just as a 2U server consumes two rack units, a 4U server consumes four rack units.

Occasionally, you will run into 2U or 4U half-rack servers. A half-rack server consumes the full rack height that a 2U or a 4U server typically requires but occupies only half the normal width. As such, two half-rack servers can be installed side by side within a single rack.

Processor support
When purchasing a 2U or 4U server, one of a manager’s first considerations is a server’s processing capabilities. Nearly all 2U servers on the market offer two sockets. Although all 2U servers support multicore processors, different servers support different processor architectures.

Intel-based 2U servers typically use 5000 series Xeon processors. For example, the Fujitsu Primergy RX300 S4 offers twin Xeon sockets that can accommodate Xeon 5200 or 5400 processors. Similarly, the HP ProLiant DL170e G6 also offers dual Xeon 5500 series processors.

It appears that 5000 series Xeon processors are becoming the standard for 2U servers, as Super Micro Computer Inc.’s 6025B-8R+V / 6025B-8R+B and Silicon Graphics International’s SGI C2108-TY10 servers use them. But Xeon processors aren’t the only option; the Dell Inc’s PowerEdge R715 uses AMD Opteron 6100 series processors, for example.

4U servers typically offer higher-grade processors such as the Intel Xeon 7000 series. For example, the Fujitsu Primergy RX600 S4 uses Intel’s Xeon 7000 series CPUs and the SGI Altix UV uses the Xeon 7500 series. Of course, you can’t assume that a 4U server automatically includes higher-end CPUs than 2U servers do. The HP ProLiant DL370 G6 and the Super Micro 6046T-TUF, for example, use Xeon 5500 and 5600 series CPUs, which are similar to those found in 2U servers. Numerous 4U servers, such as the Dell PowerEdge R905, also use AMD Opteron processors with up to six cores each.

Another difference between 2U and 4U servers is the total number of supported cores. 2U servers, such as those outlined in Table 1, usually support eight to 12 cores. But 4U servers usually offer many more cores. In fact, the SGI Altix UV 10 supports up to 32 cores. There are, however, exceptions to every rule. The Super Micro 6046T-TUF 4U server offers a mere eight cores.

Memory support
Today, most physical servers host several virtual servers. Since server virtualization is so memory-intensive, organizations want servers that support an enormous amount of memory. When memory capacity is a critical consideration, 2U and 4U servers are a good choice. It is rare to find a 2U server that supports less than 48 GB of memory, and many 2U servers support well more than 100 GB of memory. The Dell PowerEdge R715 supports up to 256 GB of memory, which is in line with what a 4U server typically offers memory-wise.

Although the total memory capacity is important for a 2U or 4U server, other aspects of server memory are worth considering. For example, look for servers that support error-correcting code (ECC) memory. Nearly all 4U servers and many 2U servers support ECC memory servers. The advantage with this type of memory is that it can correct single-bit memory errors and can detect double-bit memory errors.

Single Device Data Correction (SDDC) is another memory-integrity technology that is commonly found on 2U and 4U servers. SDDC is an extension of ECC that detects and corrects memory faults at the chip level. The process works by scattering data across multiple chips and using redundancy in a manner similar to a RAID 5 disk array. That way, even in the event of an entire chip failure, data is preserved.

Another fault-tolerant technology that is commonly found in servers is hot-spare memory. Servers like the Dell PowerEdge R905 offer a spare memory bank that can be used in the event that a memory failure occurs.

Drive support
2U and 4U servers are almost always equipped with a DVD drive for loading software, and all include some form of internal storage. When choosing a server, it is critical to consider storage needs. Most 2U and 4U servers exploit the extra chassis space to allow for multi-drive storage arrays. But the number of available drive bays, and the total amount of storage that a server can accommodate varies widely among server types.

For example, the Dell PowerEdge R905 provides only five drive bays, despite the fact that it is a 4U server. These bays provide a maximum of 2 TB of storage using 400 GB serial-attached storage (SAS) drives. In contrast, the HP ProLiant DL370 G6 can accommodate up to 24 2.5-inch drives or up to 14 3.5-inch drives with a maximum capacity of 48 TB and 28 TB, respectively, though the server has roughly the same physical dimensions as those of the Dell server.

Storage capacities differ widely among server types for two reasons. First, 2U and 4U servers are usually used in enterprise environments. Data from high-demand enterprise applications is often stored on a storage area network (SAN) or in the cloud. As such, servers that host such applications may not require much internal storage.

Second, storage capacity varies because of the different types of drives that are used. 2U and 4U servers are typically designed to support either SAS or SATA drives. SAS drives are often the drive of choice in high-demand environments because they deliver better performance than SATA drives. The tradeoff, though, is that SAS drives have much smaller capacities than their SATA counterparts.

Regardless of the drive type you choose to use, make sure that the server supports hot-swappable storage. That way, you can replace a failed drive without having to shut down the server. Remember that many servers provide additional drive controller connections on the server's main board, and additional drives attached to internal controllers may not be hot-swappable.

Expansion slots
2U and 4U server manufacturers usually make use of the server’s large physical size by providing multiple expansion slots. These slots can provide the server with additional network ports, RAID controllers, Fibre Channel cards, 1 Gigabit Ethernet (GbE) controllers, 10 GbE controllers, and so on. Expansion cards fall into two form factors: PCIe (PCI Express) and PCI-X (PCI Extended). Although these cards have similar names, they are not interchangeable.

As you shop for a server, consider not only which types of expansion cards are supported, but also the number of available channels in each slot and the PCIe version that is supported. For example, the specs on the Super Micro 6046T-TUF indicate that it offers four “x8” PCIe 2.0 ports, one “x4” PCIe 2.0 port, and one (older) “x4” PCIe port. In other words, even though this server has six PCIe expansion slots, four of those are eight-channel slots that support PCIe 2.0 cards, one is a four-channel PCIe 2.0 slot, and another is a four-channel slot for older PCIe cards.

If you purchase a server knowing that you need a certain number of expansion slots, ensure that all expansion slots are available. Some manufacturers use hardware that consumes some of the expansion slots. For example, the Fujitsu Primergy RX300 S4 technically has six PCIe slots, but one of those slots is taken up by the server’s RAID controller.

External 2U/4U server characteristics

Although it may seem obvious that every server requires power, power supply is a particularly important consideration for 2U and 4U servers. Lower-wattage power supplies may prove inadequate for servers with multiple processors, large amounts of memory, numerous hard drives and lots of expansion cards. For example, Dell offers options in the PowerEdge R715 server: Customers can choose between a 750-watt and a 1,100-watt power supply. Note that 2U and 4U servers often use much higher-wattage power supplies than 1U servers, so make sure not to overload your breakers by plugging in too many larger servers.

Other than the wattage, the most important consideration is whether a power supply is redundant. Servers that are equipped with redundant power supplies can keep running even if one power supply fails.

It is now common to connect servers with redundant power supplies to two different breakers. This way, if a breaker is tripped, the server can continue to function, as the other breaker continues to provide the server with power.

Network connectivity
At one time, network connectivity was a relatively minor concern. As long as a server had a network port or two, that was all that really mattered. Today, however, technologies such as server virtualization and failover clustering have driven the need for additional network ports.

2U and 4U servers almost always include either two or four integrated GbE ports available from the front or back panel of the server. Some manufacturers have begun to include 10 GbE ports on high-end servers. If you require additional network ports or ports of a different type (such as Fibre Channel), you have to add controllers to the server’s expansion slots.

If you do add cards to your server, you may want to make use of network interface cards with TCP/IP offloading (TOE) capabilities. These cards perform the assembly and disassembly of TCP/IP packets at the hardware level to reduce the CPU’s workload.

User interface ports
Most 2U and 4U servers can be connected directly to a keyboard, video display and mouse (KVM) for local control of the system by an administrator. When numerous servers are controlled through a central console, servers can be attached to a KVM switch, allowing a technician to physically switch a single KVM among various systems. KVM cables have a limited range, so an administration console is usually located within 50 feet of the server.

Of course, organizations are no longer limited to using traditional KVM switches. KVM-over-IP technology has been around for several years and allows a server to be managed remotely without regard for the distance between the administrator and the server.

Just as most 2U and 4U servers have KVM ports, nearly all servers also include Universal Serial Bus (USB) ports. USB ports can be used for attaching local USB keyboards and mice, external storage devices (such as DVD drives or USB hard drives), or dongles to enforce software licenses. Interface ports may be located on the front or rear of the server, or maybe both.

Controls and indicators
In addition to basic controls such as power and reset buttons, 2U and 4U servers provide a variety of external indicators. For example, 2U and 4U servers may include power, hard drive activity, network activity and system ID indicators. Some servers may also provide health indicators, such as lights that warn you if one of the server’s fans has failed. Other servers, such as the HP ProLiant, include a single LED that flashes certain patterns indicating the server’s health. While the utility of these indicators may be limited, they can assist a technician in quickly locating troubled systems and initiating specific troubleshooting steps to better identify and correct problems.

Management features for 2U/4U servers
For any modern data center, management is a major concern, especially when a single physical server hosts myriad VMs. Generally speaking, any 2U or 4U server that you purchase should have hardware-level management features, but should also support software management.

Hardware-based management features
Hardware-level monitoring capabilities exist so that administrators can monitor server health regardless of the operating system that runs on the server. One of the most common hardware management technologies is the Intelligent Platform Management Interface (IPMI), which is used by the Dell PowerEdge R715, the SGI C2108-TY10 and the Super Micro 6025B-8R+V, just to name a few.

IPMI uses a dedicated low-bandwidth network port to communicate the server’s status to IPMI-compliant management software. Because IPMI works at the hardware level, the server can communicate its status regardless of what runs on the server. In fact, because IPMI works independently of the main processor, the server does not even have to be turned on for IPMI to work. As long as a server is connected to a power source, the IPMI hardware can do its job.

IPMI is designed to monitor the state of various sensors that are built into the server. For example, server manufacturers often include sensors for monitoring temperature, voltage and fan speed. Some server manufacturers even include IPMI sensors that are designed to detect someone opening the server’s case.

Software Based Management Features
Although most servers offer hardware-level management capabilities, each server manufacturer also provides its own management software (although sometimes at an extra cost). For example, Dell Inc. has a management application called OpenManage, while Hewlett-Packard (HP) Co. provides a management console known as the HP Systems Insight Manager (SIM). Hardware management tools tend to be diagnostic in nature, while software-based tools also provide configuration and monitoring capabilities. So you might use a software management tool to configure a server’s storage array and track the consumption of computing resources.

As a general rule, hardware management is fairly standardized. Multiple vendors support an IPMI and baseboard management controller (BMC), which is another hardware management standard. Some servers, such as the Dell PowerEdge R715, support both. Management software, on the other hand, is vendor-specific. You can’t use HP SIM, for example, to manage a Dell server.

So in large data centers, server management gets complicated because of the proliferation of management software. As such, some organizations use servers from a single manufacturer to ease the management burden. Some organizations turn to third-party management software tools that support heterogeneous hardware platforms, though third-party tools often lack the granularity of control that vendor-specific tools usually offer. *

A summary of 2U and 4U servers

There are countless 2U and 4U servers on the market. Table 1 displays a representative sampling of some of the 2U servers that are currently available, while Table 2 lists a variety of 4U servers. Note that these tables do not display all of the servers that are available, even from a single vendor. Furthermore, most server vendors provide numerous configuration options, so the configurations outlined in the table may differ from what you encounter in the real world.

Table 1: Examples of 2U servers

Product Dell PowerEdge R715
Processor support Two-socket, AMD Opteron 6100 series processor
Maximum cores 24
Chipset AMD (SR5650, SR5670 and SP5100)
Memory support Up to 256 GB (16 DIMM slots): 1 GB/2 GB/3 GB/8 GB/16 GB, up to 1333 MHz
Hard drive support

2.5-inch SATA SSD, SAS (10K, 15K), Near-Line SAS (7.2K), SATA (7.2K)

Maximum internal storage: 6 TB

Expansion slots

Six PCIe G2 slots + one storage slot

Five x8 slots

One x4 slot

One x4 storage slot

Network ports Two dual-port Broadcom 5709C gigabit network interface cards (NICs) (four ports total)

Dell OpenManage

BMC, IPMI 2.0 compliant

iDRAC6 Express, with optional upgrade to iDRAC6 Enterprise and vFlash

Power supplies Support for hot-pluggable 750 W or 1100 W redundant or non-redundant power supplies


Product Fujitsu Primergy RX300 S4
Processor support Dual-socket Intel Xeon 5200 or 5400
Maximum cores Eight
Chipset Intel 5000P
Memory support 48 GB max (4 GB ECC, DDR2 SDRAM Fully buffered DIMMx12, PC2 5300F)
Hard drive support Six x 450 GB SAS drives
Expansion slots

Six low profile PCIe only (x4, x8) (SAS RAID Controller occupies one slot)

One PCI-X (64-bit/133 MHz)

Network ports Dual GbE ports
Manageability ServerView (Standard) management software
Power supplies 645 W with optional hot plug


Product HP ProLiant DL170e G6
Processor support Dual-socket Intel Xeon 5500 and 5600 series
Maximum cores 12
Chipset Intel Xeon 5520
Memory support Maximum (RDIMM) 192 GB (12 x 16 GB PC3-8500 R) (DDR3-800)
Hard drive support Maximum 16 TB internal storage – LFF 3.5-inch hot plug SATA or SAS 8 x 2 TB
Expansion slots Three PCIe (x8) ports
Network ports HP NC362i integrated dual-port gigabit server adapter
Manageability HP iLO Advanced
Power supplies HP Common Slot Power Supply bay


Product SGI C2108-TY10
Processor support Two Intel Xeon quad-core or six-core 5500 or 5600 series processors
Maximum cores 12
Chipset Intel 5520
Memory support

18 slots providing a maximum of 144 GB

1333/1066/800 MHz DDR3 ECC reg. or unbuffered

Hard drive support

Eight 3.5-inch SAS or SATA II hot-swappable bays

Supports up to 16 TB of storage

Expansion slots

Two PCIe 2.0 x8

One PCIe x4

Two PCI-X 133

Network ports Dual GigE (Intel 82576)
Manageability Integrated IPMI 2.0
Power supplies 720 W redundant auto-switching


Product Super Micro 6025B-8R+V / 6025B-8R+B
Processor support Dual Intel 64-Bit Xeon dual-core or quad-core CPUs
Maximum cores Eight
Chipset Intel Xeon 5000 series
Memory support 16 240-pin DIMM supporting up to 64 GB (667 / 533 MHz)
Hard drive support Eight 3.5-inch hot-swappable drive bays
Expansion slots

Two (x8) PCIe

One (x4) PCIe (using an x8 slot)

Two PCI-X 64-bit 133 MHz

One PCI-X 64-bit 100 MHz

Network ports 2x RJ45 ports
Manageability IPMI 2.0
Power supplies Redundant 700 W


Table 2: Examples of 4U servers

Product Dell PowerEdge R905
Processor support Four-socket , six-core AMD Opteron processors
Maximum cores 24 (six cores per socket)
Chipset Broadcom HT-2100 and HT-1100 server
Memory support

Up to 256 GB (32 DIMM slots): 512 MB/1 GB/2 GB/4 GB/8 GB ECC DDR2 SDRAM

Single- or dual-rank DIMM Support

ECC and SDDC technologies maintain data integrity and help to prevent single bit or multi bit errors on DIMMs

Spare Bank support for enhanced data protection in the event of a DRAM chip failure (requires 16 or 32 DIMMs populated)

Hard drive support Up to 2 TB via five x 3.5-inch 400 GB hot-plug SAS hard drives
Expansion slots Seven PCIe G1 slots; two x8 slots; five x4 slots
Network ports Four Broadcom NeXtreme II 5708 GbE NICs with failover and load balancing TOE


Standard Baseboard Management Controller with IPMI 2.0 support

Optional DRAC 5

Power supplies Redundant 1100 W hot-pluggable power supplies


Product Fujitsu Primergy RX600 S4
Processor support Four-socket Intel Xeon 7000 series
Maximum cores Up to 24
Chipset Intel Xeon 7300
Memory support Maximum 128 GB (4 GB ECC DDR2 SDRAM fully buffered DIMM x32 PC2 5300F)
Hard drive support Eight hot-pluggable drive bays 2.5-inch SAS drives with a maximum size of 1174.4 GB
Expansion slots

PCIe (x8) four hot-pluggable slots

PCIe (x4/x8) three non hot-pluggable

Network ports Four GbE ports
Manageability ServerView Standard management software
Power supplies Redundant hot-pluggable power supply up to 1215 W depending on the CPU configuration


Product HP ProLiant DL370 G6
Processor support Dual-socket Intel Xeon dual-core and quad-core
Maximum cores Eight
Chipset Intel Xeon 5500 series
Memory support 192 GB maximum
Hard drive support Supports up to 24 2.5-inch drives or 14 3.5-inch drives; SATA and SAS are supported
Expansion slots Nine PCIe 2.0 Express slots
Network ports HP NC375i quad-port GbE adapter
Manageability HP Systems Insight Manager and HP ProLiant Onboard Administrator
Power supplies Optional redundant power supply


Product SGI Altix UV 10
Processor support Four-socket Intel Xeon quad-, six- or eight- core
Maximum cores 32
Chipset Intel Xeon 7500 series
Memory support One TB max, 64 x 1066 MHz DDR3 ECC reg.
Hard drive support Eight 2.5-inch SAS or five 2.5-inch SATA II hot-swappable bays.
Expansion slots

One PCIe 2.0 x16

Four PCIe 2.0 x8

One PCIe 2.0 x4

Two PCIe x4

Network ports Four GbE (dual Intel 82576)
Manageability IPMI 2.0
Power supplies Four 850 W 3+1 redundant auto-switching


Product Super Micro 6046T-TUF
Processor support Dual-socket 64-bit Intel Xeon processors
Maximum cores Eight
Chipset Intel Xeon 5500 and 5600
Memory support 12x 240-pin DIMM sockets for a maximum of 192 GB (1333/1066/800 MHz DDR3 ECC registered memory)
Hard drive support 5x 3.5-inch hot-swappable SATA bays
Expansion slots

Four (x8) PCIe 2.0

One (x4) PCIe 2.0

One (x4) PCIe

Network ports Dual Intel 82574L GbE controllers
Manageability IPMI 2.0
Power supplies 865 W redundant


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ABOUT THE AUTHOR: Brien M. Posey has received Microsoft’s Most Valuable Professional award six times for his work with Windows Server, IIS, file systems/storage and Exchange Server. He has served as CIO for a nationwide chain of hospitals and healthcare facilities and was once a network administrator for Fort Knox.

What did you think of this feature? Write to's Nicole Harding about your data center concerns at [email protected].


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