Windows boot IO and storage performance impact on VDI

By Greg Schulz, Server and StorageIO @storageio

With Virtual Desktop Infrastructures (VDI) initiatives adoption being a popular theme associated with cloud and dynamic infrastructure environments a related discussion point is the impact on networks, servers and storage during boot or startup activity to avoid bottlenecks. VDI solution vendors include Citrix, Microsoft and VMware along with various server, storage, networking and management tools vendors.

A common storage and network related topic involving VDI are boot storms when many workstations or desktops all startup at the same time. However any discussion around VDI and its impact on networks, servers and storage should also be expanded from read centric boots to write intensive shutdown or maintenance activity as well.

VDI boot and shutdown storm clouds
Boot and shutdown storms are the result of many virtual desktops either starting up (boot) or shutting down causing many IO operations (IOPS) to be performed in a short amount of time. The reason that they are referred to as boot (more reads) or shutdown (many writes) storms is that a single or couple of workstations doing IOPS to their local hard disk drive (HDD), hybrid HDD (HHDD) or solid state device (SSD) have no significant impact on networks or servers and their storage. Other than making requests of servers for network resources, most of the IOPS associated with the boot or shutdown are localized to the workstation or laptop HDD, HHDD or SSD.

When the functionality supported by the local HDD, HHDD or SSD is moved in a VDI, those IOPS that would otherwise have been resolved locally now must come from a server and its storage that are accessed over a network. For example a lightly loaded or configured windows workstation, laptop or desktop could have as low as 30 IOPS per second during a boot which does not seem like a lot for a typical hard disk drive. After all, an average 7,200 (7.2K) Revolution per minute (RPM) HDD should be good for about 125 to 150 small IOPs per second depending of course on the type of adapter, controller and other configuration options. A fast 15K SAS or Fibre Channel HDD should be good for around 200 to 250 IOPS again depending upon the configuration, what it is attached to, workload and other factors.

VDI VMsA few workstations, desktops or laptops booting or shutting down at the same time will put some extra traffic on the network, servers and shared storage, however the exact impact will vary based on a couple of factors. The challenge with VDI environment is if you take 10 or 20, perhaps 100 or 200 or more desktops and move their disk data to a server, which also means that the IOs will be redirected from a local HDD over a network to storage attached to a server via internal DAS, shared SAS DAS, SAN or NAS. What this means is more traffic on the network plus consolidating disk IOs on the server and its storage capabilities.

Workstation or desktop impact factors on physical and VDI environments include:

  • The number and type of workstations, desktops or laptops that exists
  • What their operating system is and its configuration
  • How many and what types of applications are installed
  • Existence of any protocol, bandwidth or network optimization
  • Type, speed and configuration of networks that are in use

hIOmon, how many IOPs does your workstation or desktop do during a boot?
There are different rules of thumb (RUTs) and metrics floating around as to how many IOPS a typical windows desktop requires when booting. The reason the focus on windows is that the overall majority or most commonly virtualized desktop (as well as server) are Microsoft based products onto either Citrix, Microsoft Hyper-V or VMware vSphere among other hypervisors. Having heard different RUTs being tossed around ranging from 30 IOPS which seemed rather low, to a more conservative 50 to 100 IOPS (your activity will vary), I decided it was time for some actual measurements and metrics.

Realizing that standard windows performance tools along with clocks for timing were not going to be easy or give me what I wanted to see, I decided to download a tool I know of to do just this activity. That tool is called hIOmon from Tom West and hyperIO which once installed provides some very detailed and interesting measurements of Windows environments down to granular individual file access activity, process and other information. If you are a current or former performance and capacity planning analysis and have been told about how great mainframes are or were because of their SMF and RMF information, you can get related information out of Windows with tools such as hIOmon or even Unix and other systems for that matter (go check out if you are not aware of them).

By the numbers: IO, IO its of to hIOmon and work I go
I installed a copy of hIOmon on four different desktops including a mix of laptops and tower based workstations, all running Windows 7 Ultimate 32 bit. The two laptops each has a HHDD which include a 4GB SLC (single level cell) flash (e.g. SSD) integrated inside the 7,200 RPM 500GB HDD that support many different common productivity applications and tools. One of the laptops is a stripped down Windows 7 Ultimate using an older and slower traditional HDD and no applications other than iTunes installed on it. The mini tower workstation (results are not shown) also had an older HDD with similar applications and tools configuration as the two laptops. In the course of testing the mini tower which was recently upgrading from Windows XP SP3 to Windows 7, I uncovered some things that will result in upgrades being applied in the next few days including more DRAM and replacing the HDD with an HHDD.

Industry Trend

With hIOmon installed and after doing a couple a shutdown, then restart to make sure everything was normal on the systems and that no windows or other software updates were being, or scheduled for installation, the testing began. The test was to simply shut down the workstations to a cold state, then with stopwatch start timing when the boot began noting the time at which I was able to login and the time at which Windows appeared to be stable. As an indicator, I used the small circle Icon (or whatever you may have changed it to) on Windows 7 to show that most if not all the startup activity has stabilized.

Using hIOmon reporting tool (see screen shots below), I then displayed the performance data for the time interval up to the noted windows stabilization time. These metrics for the boot device included read and write IOPs, bytes read and written along with read and write latency (ms) from which I was able to compute IO per second, throughput or bandwidth per second along with other statistics. I also recorded for each test run what the metrics were about four minutes after the start of the boot process to decide how much other background activity continued after the first startup time.

hIOmon WMI browser selecting device performance metrics
hIOmon Metrics
hIOmon device performance metrics from a 120 second duration

The following table shows a summary of my informal test results confirming that your desktop boot (as well as shutdown) performance will vary. I saw a range of different IOPs with the HHDD having a higher number which should not be a surprise given that they do more consolidated work in a shorter period. Likewise by looking at the data collected during different time intervals will result in various results such as measuring during the peak of the boot process vs. waiting a minute or two for background tasks to quiet down.

HHDD desktop 1
HDD desktop 1
HHDD desktop 2
69 to 113
186 to 353
Avg. MByte sec
1.58 to 2.13
2.76 to 5.2
Percent IOPS read
80 to 88%
Percent MBs read
63 to 77%
Mbytes read
201 to 245
Mbytes written
60 to 141
Avg. read latency
8.2 to 9.5ms
Avg. write latency
20.5 to 14.96ms
Boot duration
120 seconds
120 to 240 sec

Note that for HHDD workstation 2 results shown above, metrics were collected at both 120 and 240 seconds from start of boot. The lower result numbers are based on the results at 240 seconds after much of the heavy lifting was done, with the larger metrics at 120 seconds. What the quick testing showed is that depending upon the type of desktop, configuration, the number and types of applications and data that are loaded during boot (or saved during shutdown) will have an impact on performance.

The above table shows that having insight into your environments applications and desktops as a baseline is important for making informed decisions on resource planning including what network enhancements, servers and storage will be needed for VDI or other improvements. Another benefit of having a baseline performance across different classes of applications and desktops is to be able to compare before and after, or to decide when there is a problem how the resources are performing compared to a normal mode. Also looking at the above table, the benefits of a faster local device in the form of a HHDD or a SSD can be seen which means for moving to a VDI environment, a fast low latency network able to move the required amount of data is needed along with fast servers and back end storage. In other words, fast desktops need fast networks, servers and storage.

Not surprisingly I saw the desktop with fewer applications and a smaller software footprint need or generate less IOPs (along with bytes read and written) during the boot process, not to mention boot in relatively short amount of time. The two desktops with HHDDs and many applications installed accomplished their boot in a shorter amount of time, in fact each success cold boot (from machine turned off) tended to be faster given the built in caching of the integrated SLC (single layer cell) flash (SSD) that is part of the HHDD. However, looking at the number of IOPS and bytes read and written during the interval, of course there were more. What this illustrated was that the HHDD (SSDs should give similar capabilities) were able to handle the increased activity in a shorter amount of time, however it also gave an indicator of either how many IOPs, how many bytes read and written, along with elapsed wall time or boot duration if that activity were placed onto a network.

General comments
Since I use Hybrid Hard Disk Drives (HHDD) such as the Seagate Momentus XT in some of my workstations, the benefit of integrated SLC flash memory as a cache combined with 7.2K 500GB SATA HDD became clear over next cold boots (e.g. complete power off to start up) tests. The benefit was that each successive boot was faster improving on average by several seconds. Other impacts and variables include IO optimization techniques for your physical and VDI including file system and application tuning, network, server and storage load balancing, bandwidth and protocol optimization among others.

Number of IOPs may be lower if your boot (or shutdown) time is longer on a HDD vs. HHDD or SSD where more work is done in a shorter period. Likewise, your number of IOPS may be higher if using HHDD and SSD with a shorter elapsed time. Reads tend to be easier for a device or server to handle as they are more cache friendly. For boots, there will be some writes, however for shutdown that activity will be amplified. Measure networks, servers and storage in the context of performance (IOPS, throughput or bandwidth and latency), capacity (space), availability (HA, RAID, BC, backup), and economics along with energy effectiveness.

Additional tips, comments and recommendations

  • Gain situational awareness with metrics and measurement for End to End (E2E) management
  • Avoid making apples to oranges comparison with regards to metrics and desktop configuration
  • Boot storms are a focus, however also make sure your VDI is optimized for shutdown as well
  • There are many variables and thus your mileage or IOPs will vary
  • How you measure the workstation performance will have an impact as will how you collect metrics
  • What time interval you use such as when is the boot done
  • Look beyond the number of IOPs of a desktop also consider the total number of bytes read and written
  • Keep an eye on latency as well as IOPS, packets and frames per second on the network
  • Avoid simply moving problems, find and fix them, then load balance or move as applicable
  • Reduce your data footprint impact with data footprint reduction (DFR) techniques and tools
  • Look into application virtualization and presentation services to off loading desktops
  • The above will have an impact on networks, servers and their shared (or dedicated) storage
  • The best benchmark is your real applications being measured by applicable metrics
  • Aggregation can cause aggravations, not all applications or functionality can be consolidated, however most can be virtualized
  • Look at virtualization in the context of life beyond consolidation
  • Similar to clouds, with VDI dont be scared, however look before you leap and be prepared

What is your take, what are you seeing for Windows boot and shutdown IO performance impact or characteristics?

I invite you to try hiMON and post what you are seeing given your workstations and or servers configuration profiles.

Also check out the new hIOmon Disk IO ranger tools that Tom West has posted on the hyperIO site.

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