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Windows Server 2012 : Comprehensive Performance Analysis and Logging (part 6) - Resolving disk I/O bottlenecks, Resolving network bottlenecks

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Resolving disk I/O bottlenecks

With the high-speed disks available today, a system’s hard disks are rarely the primary reason for a bottleneck. It is more likely that a system is having to do a lot of disk reads and writes because there isn’t enough physical memory available and the system has to page to disk. Because reading from and writing to disk is much slower than reading and writing memory, excessive paging can degrade the server’s overall performance. To reduce the amount of disk activity, you want the system to manage memory as efficiently as possible and page to disk only when necessary.

That said, you can do several things with a system’s hard disks to improve performance. If the system has faster drives than the ones used for the paging file, you might consider moving the paging file to those disks. If the system has one or more drives that are doing most of the work and other drives that are mostly idle, you might be able to improve performance by balancing the load across the drives more efficiently.

To help you better gauge disk I/O activity, use the following counters:

  • PhysicalDisk\%Disk Time Records the percentage of time the physical disk is busy. Track this value for all hard disk drives on the system in conjunction with Processor\%Processor Time and Network Interface Connection\Bytes Total/Sec. If the %Disk Time value is high and the processor and network connection values aren’t high, the system’s hard disk drives might be creating a bottleneck. You might be able to improve performance by balancing the load across the drives more efficiently or by adding drives and configuring the system so that they are used.

    Note

    Redundant array of independent disks (RAID) devices can cause the PhysicalDisk\%Disk Time value to exceed 100 percent. For this reason, don’t rely on PhysicalDisk\%Disk Time for RAID devices. Instead, use PhysicalDisk\Current Disk Queue Length.

  • PhysicalDisk\Current Disk Queue Length Records the number of system requests that are waiting for disk access. A high value indicates that the disk waits are affecting system performance. In general, you want there to be very few waiting requests.

    Note

    Physical disk queue lengths are relative to the number of physical disks on the system and proportional to the length of the queue minus the number of drives. For example, if a system has two drives and there are 6 waiting requests, that could be considered a proportionally large number of queued requests; but if a system has eight drives and there are 10 waiting requests, that is considered a proportionally small number of queued requests.

  • PhysicalDisk\Avg. Disk Write Queue Length Records the number of write requests that are waiting to be processed.

  • PhysicalDisk\Avg. Disk Read Queue Length Records the number of read requests that are waiting to be processed.

  • PhysicalDisk\Disk Writes/Sec Records the number of disk writes per second. It is an indicator of how much disk I/O activity there is. By tracking the number of writes per second and the size of the write queue, you can determine how write operations are affecting disk performance. If lots of write operations are queuing and you are using RAID 5, it could be an indicator that you would get better performance by using RAID 1. Remember that by using RAID 5 you typically get better read performance than with RAID 1. So, there’s a tradeoff to be made by using either RAID configuration.

  • PhysicalDisk\Disk Reads/Sec Records the number of disk reads per second. It is an indicator of how much disk I/O activity there is. By tracking the number of reads per second and the size of the read queue, you can determine how read operations are affecting disk performance. If lots of read operations are queuing and you are using RAID 1, it could be an indicator that you would get better performance by using RAID 5. Remember that by using RAID 1 you typically get better write performance than RAID 5. So, as mentioned, there’s a tradeoff to be made by using either RAID configuration.

Resolving network bottlenecks

The network that connects your computers is critically important. Its responsiveness, or lack thereof, weighs heavily on the way users perceive the responsiveness of their computers and any computers to which they connect. It doesn’t matter how fast their computers are or how fast your servers are. If there’s a big delay (and big network delays are measured in tens of milliseconds) between when a request is made and the time it’s received, users might think systems are slow or nonresponsive.

Unfortunately, in most cases, the delay (latency) users experience is beyond your control. It’s a function of the type of connection the user has and the route the request takes to your server. The total capacity of your server to handle requests and the amount of bandwidth available to your servers are factors you can control, however. Network capacity is a function of the network cards and interfaces configured on the servers. Network bandwidth availability is a function of your organization’s network infrastructure and how much traffic is on it when a request is made.

Counters you can use to check network activity and look for bottlenecks include the following:

  • Network Interface\Bytes Total/Sec Records the rate at which bytes are sent and received over a network adapter. Track this value separately for each network adapter configured on the system. If the Bytes Total/Sec for a particular adapter is substantially slower than what you’d expect given the speed of the network and the speed of the network card, you might want to check the network card configuration. Check to see whether the link speed is set for half duplex or full duplex. In most cases, you’ll want to use full duplex.

  • Network Interface\Current Bandwidth Estimates the current bandwidth for the selected network adapter in bits per second. Track this value separately for each network adapter configured on the system. Most servers use 100-Mbps, 1-Gbps, or 10-Gbps network cards, which can be configured in many ways. Someone might have configured a 1-Gbps card for 100 megabits per second (Mbps). If that is the case, the current bandwidth might be off by a factor of 10.

  • Network Interface\Bytes Received/Sec Records the rate at which bytes are received over a network adapter. Track this value separately for each network adapter configured on the system.

  • Network Interface\Bytes Sent/Sec Records the rate at which bytes are sent over a network adapter. Track this value separately for each network adapter configured on the system.

TROUBLESHOOTING: Compare network activity to disk time and processor time

Compare these values in conjunction with PhysicalDisk\%Disk Time and Processor\%Processor Time. If the disk time and processor time values are low but the network values are very high, a capacity problem might exist. Solve the problem by optimizing the network card settings or by adding an additional network card.

You might be able to improve network performance by installing multiple network adapters and teaming the network cards. You configure NIC teaming using Server Manager by selecting Local Server in the left pane and then tapping or clicking the link provided for NIC teaming. You can then create and configure NIC teams.

INSIDE OUT: NIC teaming

NIC teaming allows multiple network adapters to have their bandwidth aggregated for the purposes of load balancing and failover protection. Windows Server 2012 supports up to 32 network adapters aggregated into a team. In turn, these aggregated adapters then present one or more virtual adapters, referred to as team network adapters, to the operating system. Each team network adapter organizes network traffic by virtual LAN (VLAN), allowing applications to simultaneously connect to different VLANs.

When you are configuring NIC teaming, you can tap or click Additional Properties to configure the teaming mode, load-balancing mode, and standby-adapter mode. By default, team network adapters use the switch independent team mode, which doesn’t require the network switch to participate in the teaming, and this allows the team network adapters to be connected to different switches. Alternatively, you can configure

  • Static teaming as the teaming mode, which requires you to configure the switch and the server to work with NIC teaming. Here, you typically use a server-class switch and identify which links form the team. Because there is no error detection and correction, you must be certain the network cables are properly connected.

  • Link Aggregation Control Protocol (LACP) as the teaming mode, which uses Institute of Electrical and Electronics Engineers (IEEE) 802.1ax LACP to automatically create the NIC team by dynamically identifying links between the server and the switch. Here, you typically use a server-class switch and enable LACP on the appropriate switch ports.

If you have a server-class switch, the switch likely supports IEEE 802.1ax (also referred to as IEEE 802.3ad) and you can gain some additional performance benefits by having the switch participate in the teaming.

For load balancing, the default mode (Address Hash) creates a simple hash for packets and then assigns packets that have a particular hash to one of the available team network adapters. This can help to balance the workload across the team network adapters. Alternatively, if a server has virtual machines, you can use the MAC address of each virtual machine to determine how traffic is balanced. Load balancing by MAC address works best when virtual machines have similar workloads. Keep in mind that failover between network adapters in a virtual machine might result in traffic being sent with the MAC address of a different network adapter. If so, to prevent this from being blocked automatically, NIC teaming must be set to allow MAC spoofing or must have the “AllowTeaming=On” parameter set using the Set-VmNetworkAdapter cmdlet in Windows PowerShell.

Finally, the Standby Adapter setting allows you to specify whether all network adapters are active. Typically, for optimal performance, you’ll want all network adapters in a team to be active. However, you can designate one or more network adapters in a team as standby adapters. Exactly as its name suggests, a standby adapter is inactive until another active adapter fails and is then activated as part of failover. Keep in mind that, technically, you can place a single network adapter in a team. However, you need two or more network adapters for fault protection through failover.

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