Manual Chapter :
About load balancing
and resource availability
Applies To:
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BIG-IP DNS
- 14.1.3, 14.1.2, 14.1.0
About load balancing
and resource availability
BIG-IP DNS load balances DNS name
resolution requests to resources based on availability. A resource is available when it meets one
or more pre-defined requirements. BIG-IP DNS uses three methods to determine resource
availability: a dependency on another resource, limit settings, or a set of values returned by a
monitor. When BIG-IP DNS considers a resource unavailable, BIG-IP DNS attempts to select the next
resource based on the current load balancing method.
About virtual server dependency
Within BIG-IP
DNS, you can configure a virtual server to be available based on the
availability of other virtual servers.
Consider the fictional company SiteRequest. One of the servers, serverMain,
at the Tokyo data center has two virtual servers: vsContact, which points to the contacts page of
the web site, and vsMail, which points to the mail system. The vsMail virtual server is in the
Dependency List of the vsContact virtual server. As a result, BIG-IP DNS considers the vsContact
virtual server available only if the vsMail virtual server is also available.
Configuring virtual
server availability to be dependent on the status of other virtual servers
Ensure that multiple virtual servers are configured on the server. Determine the
virtual servers upon which you want the availability of a virtual server to be
dependent.
Configure a virtual server to be available based
on the availability of other virtual servers by configuring a
Dependency List
for the virtual
server. - On the Main tab, click.The Server List screen opens.
- In the Server List area, click a server name.The server settings and values display.
- On the menu bar, clickVirtual Servers.A list of the virtual servers configured on the server displays.
- In the Virtual Servers list, click a virtual server name.The virtual server settings and values display.
- From theConfigurationlist, selectAdvanced.Additional controls display on the screen.
- In the Dependency List area, from theVirtual Serverslist, select each virtual server on which you want the virtual server to be dependent, and then clickAdd.The virtual servers display in the list as you add them.
- ClickFinished.
The virtual server is now available only when the virtual servers on the dependency
list are also available.
Limit settings for resource availability
This table describes the limit settings BIG-IP
DNS uses to determine
resource availability. A
limit setting
is a threshold for a statistic associated
with a system.Limit setting | Server-level | Pool-level | Virtual Server-level | BIG-IP Systems | Other Load Balancers | Hosts |
---|---|---|---|---|---|---|
Maximum allowable throughput in bits per second | Y | Y | Y | Y | Y | Y |
Packets | Y | Y | Y | Y | Y | Y |
Current connections | Y | Y | Y | Y | Y | Y |
Connection | N | N | Y | Y | N | N |
CPU | Y | N | N | N | Y | Y |
Memory | Y | N | N | N | Y | Y |
About wide IP-level load balancing
BIG-IP®
DNS selects pools based on the
order in which they are listed in a wide IP. When you organize pools in conjunction with the
Global Availability, Ratio, Round Robin, and Topology load balancing methods, consider the order
in which the pools are listed in the Pool List.
The Global
Availability load balancing method instructs BIG-IP DNS to select the first pool in the wide IP
pool list until it becomes unavailable, and then to select the next pool in the list until the
first pool becomes available again. This ensures that the most robust pool receives DNS name
resolution requests, while the other pools act as backups in case the primary pool becomes
unavailable.
A pool is available if it can provide an
answer from any of the configured response methods: Preferred load balancing (LB), Alternate LB,
Fallback LB, or Fallback IP.
About the Global Availability load balancing method
The
Global Availability
load balancing
method distributes DNS name resolution requests based on the order of resources in a list. Using
global availability, BIG-IP® DNS sends a request to the
first available resource in a list. Only when a resource becomes unavailable does BIG-IP DNS send
requests to the next resource in the list. You can only select the next resource in the list if
the Fallback load balancing method is set to None. Over time, the first resource in the list
receives the most requests and the last resource in the list receives the least requests.Testing global server load balancing without verifying availability of virtual servers
You can configure BIG-IP DNS load balancing in a staging environment to load balance DNS
name resolution requests to virtual servers without verifying the availability of the
virtual servers.
- On the Main tab, click.The Load Balancing configuration screen opens.
- Deselect theVerify Virtual Server Availabilitycheck box.
- ClickUpdate.
About the Ratio load balancing method
The
Ratio
load balancing method distributes DNS name resolution requests among the
virtual servers in a pool or among pools in a multiple pool configuration using weighted
round robin
, a load balancing pattern in which requests are distributed among several
resources based on a priority level or weight assigned to each resource. Using the Ratio method, you can configure BIG-IP® DNS to send twice
as many connections to a fast, new server, and half as many connections to an older, slower
server.
About wide IPs and weighting pools for the Ratio load balancing method
When you configure a wide IP to use the Ratio load balancing method, BIG-IP®DNS load balances DNS name resolution requests across the pools
in the wide IP based on the weight assigned to each pool. BIG-IP DNS uses pool weight as a
percentage of the total of the weights of all the pools in the wide IP to determine the frequency
at which a pool receives connection requests.
Consider the fictional company SiteRequest, where the wide IP
www.siterequest.com
contains three pools, with the following weight
assignments: - Pool 1: weight 50
- Pool 2: weight 25
- Pool 3: weight 25
- Pool 1: selected 50 percent of the time
- Pool 2: selected 25 percent of the time
- Pool 3: selected 25 percent of the time
About pools and weighting pool members for the Ratio load balancing method
When you configure a pool to use the Ratio load balancing method, BIG-IP®
DNS, formerly Global Traffic Manager™ (GTM™), load
balances requests across the pool members based on the weight assigned to each pool member
(virtual server). The system uses pool member weight as a percentage of the total of the weights
of all the members assigned to the pool to determine the frequency at which a pool member
receives connection requests.
Consider the fictional company SiteRequest, where the wide IP
www.siterequest.com
contains a pool named poolMain. This pool contains
three members, with the following weight assignments: - Virtual Server 1: weight 50
- Virtual Server 2: weight 25
- Virtual Server 3: weight 25
- Virtual Server 1: selected 50 percent of the time
- Virtual Server 2: selected 25 percent of the time
- Virtual Server 3: selected 25 percent of the time
About the Round Robin load balancing method
The
Round Robin
load balancing method distributes DNS name resolution requests in
a circular and sequential pattern among the virtual servers in a pool. Over time, each virtual
server receives an equal number of connections.About pool-level load balancing
BIG-IP®
DNS provides three tiers of
pool-level load balancing to identify a virtual server to handle a DNS name resolution request.
- Preferred Load Balancing Method
- The first load balancing method that BIG-IP DNS uses to return the IP address of a virtual server in response to a DNS name resolution request. The preferred method can be either static or dynamic.
- Alternate Load Balancing Method
- If the preferred load balancing method fails to return a valid resource in response to a DNS name resolution request, it is likely that BIG-IP DNS was unable to acquire the proper metrics to perform load balancing. The alternate method can be only static.
- Fallback Load Balancing Method
- If the alternate load balancing method fails to return a valid resource in response to a DNS name resolution request, BIG-IP DNS uses the fallback method. To ensure that BIG-IP DNS returns a response to a request, the fallback method ignores the availability status of a resource. The fallback method can be either static or dynamic.
- When the fallback method is set to Return to DNS, the response from BIND is processed the same as any other DNS packet. If the response from BIND matches a wide IP, the load balancing algorithm may use a downed pool. If the downed pool is not configured to select another pool based on availability of the pool members, you can select a downed pool. For this scenario, BIND is not used a second time.
If all of the configured load balancing methods fail to provide a valid resource in response to
a DNS name resolution request, either the request fails or BIG-IP DNS uses the local BIND to
resolve the request.
About the Drop Packet load balancing method
The
Drop Packet
load balancing method indicates that BIG-IP®
DNS drops a DNS name
resolution request. This load balancing method is most often selected for the Alternate load
balancing method to ensure that BIG-IP DNS does note return an IP address for an unavailable
resource.About the Virtual Server Score load balancing method
The
Virtual Server Score
load balancing method distributes DNS name resolution
requests to pool members (virtual servers) based on a user-defined ranking system.This method can be used only for distributing requests to pool members controlled by
BIG-IP®
Local Traffic Manager (LTM™) systems.
About the Virtual Server Capacity load balancing method
The
Virtual Server Capacity
load balancing method distributes DNS name resolution
requests to pool members (virtual servers) based on a system-generated list of pool members
(virtual servers) weighted by capacity. BIG-IP DNS selects the pool member with the greatest
capacity most often, but over time, all pool members are returned in responses. When pool members
have the same capacity, BIG-IP DNS uses the Round Robin method to select a pool member. About the Round Trip Times load balancing method
The
Round Trip Times
load balancing method distributes DNS name resolution
requests to the pool member (virtual server) with the fastest measured round trip time between a
data center and a client's LDNS.About the Packet Rate load balancing method
The
Packet Rate
load balancing method distributes DNS name resolution requests to
the pool member (virtual server) that is currently processing the fewest number of packets per
second.About the Least Connections load balancing method
The
Least Connections
load balancing method distributes DNS name resolution
requests to pool members (virtual servers) that are managed by load balancing servers, such as
BIG-IP®
Local Traffic Manager (LTM™). BIG-IP DNS selects a pool
member that currently hosts the fewest connections.About the Kilobyte/Second load balancing method
The
Kilobyte/Second
load balancing method distributes DNS name resolution requests to the pool member (virtual server) that is currently processing the fewest number of kilobytes per second.This method can be used only with servers for which BIG-IP DNS can collect the
kilobytes per second metric.
About the Hops load balancing method
The
Hops
load balancing method distributes DNS name resolution requests based on
the traceroute
utility and tracks the number of intermediate system
transitions (router hops) between a client's LDNS and each data center. BIG-IP DNS distributes
requests to a pool member in the data center that is the fewest router hops from the LDNS.About the Completion Rate load balancing method
The
Completion Rate
load balancing method distributes DNS name resolution requests
to the pool member (virtual server) that currently maintains the least number of dropped or
timed-out packets during a transaction between a pool member in a data center and the client's
LDNS.About the CPU load balancing method
The
CPU
load balancing method distributes DNS name resolution requests to the pool member (virtual server) that currently has the most CPU processing time available.About the Return to DNS load balancing method
The
Return to DNS
load balancing method immediately returns DNS name resolution requests to the LDNS for resolution. When you use this load balancing method, for client queries, the BIG-iP system increments the Return to DNS statistics; otherwise, the system increments the Return from DNS statistics. Use this method when you want to temporarily remove a pool from service or when you want to limit a pool, in a single pool configuration, to only one or two request attempts.
About Static Persist load balancing
The Static Persist load balancing method uses the persist mask, with the source IP address of
the LDNS, in a deterministic algorithm to send requests to a specific pool member (virtual
server). Using this method, BIG-IP®
DNS sends DNS name resolution requests to the first available pool member
based on a hash algorithm that determines the order of the pool members. This algorithm orders
the pool members differently for each LDNS that is sending requests to BIG-IP DNS, taking into account
the Classless Inter-Domain Routing (CIDR) of the LDNS. As BIG-IP DNS distributes requests across all
pool members, requests from each LDNS (and thus, each client) are generally sent to the same pool
member. When the selected pool member becomes unavailable, BIG-IP DNS sends requests to another pool
member. When the original pool member becomes available again, BIG-IP DNS sends requests to that pool
member.
About the Fallback IP load balancing method
The
Fallback IP
load balancing method distributes DNS name resolution requests to
a specific user-specified IP address. This IP address is not monitored for availability. Use this
load balancing method only for the Fallback IP method and specifically to provide a disaster
recovery site.Verifying the availability of virtual servers when using the fallback load balancing
method
You can configure BIG-IP DNS to verify that a virtual server is up before returning
the IP address of the virtual server in a response to a DNS name resolution request. Do
this when the preferred and alternate load balancing methods assigned to a pool do not
return a valid response and BIG-IP DNS begins to use the configured fallback load
balancing method.
- On the Main tab, click.The Load Balancing configuration screen opens.
- Select theRespect Fallback Dependencycheck box.
- ClickUpdate.
About the None load balancing method
The
None
load balancing method skips the current load balancing method, distributes DNS name resolution requests to the next available pool in a multi-pool configuration.If the alternate
load balancing method for a pool is None, BIG-IP DNS skips the alternate method and immediately
tries the fallback method. If the fallback method is None, and there are multiple pools
configured, BIG-IP DNS uses the next available pool. You can only select the next pool in the
list if the fallback load balancing method is set to None. If all pools are unavailable, BIG-IP
DNS returns an aggregate of the IP addresses of all pool members using BIND. Alternatively, when
the preferred method for all pools is configured, but the alternate and fallback methods are set
to None, if the preferred method fails, BIG-IP DNS uses the next available pool.
About the QoS load balancing method
The
Quality of Service (QoS)
dynamic load balancing method uses current
performance metrics to calculate an overall QoS score for each pool member (virtual server). When
load balancing DNS name resolution requests, BIG-IP DNS selects a virtual server with the best
overall QoS score. If virtual servers have identical scores, BIG-IP® DNS
load balances connections to those virtual servers using the round robin method. If QoS scores
cannot be determined, BIG-IP DNS load balances connections across all pool members using the
round robin method. Understanding the QoS equation
The equation for calculating the overall Quality of Service (QoS) score
is:
POOL_CONFIG->rtt * (GLOBALS->rtt / path->rtt) * 10 + POOL_CONFIG->hops * (GLOBALS->hops / path->hops) + POOL_CONFIG->hit_ratio * (path->hit_ratio / GLOBALS->hit_ration+ POOL_CONFIG->packet_rate * (GLOBALS->packet_rate / vs->packet_rate) * 100 + POOL_CONFIG->bps * (GLOBALS->bps / vs->bps) + POOL_CONFIG->topology * (topology_match->score / GLOBALS->topology) + POOL_CONFIG->vs_capacity * vs->cur_serv_cnt + POOL_CONFIG->vs_score * vs->cur_vs_score + POOL_CONFIG->lcs * vs->link->lcs * 10
Pool members (virtual servers) inherit the QoS settings from the pool. In the equation, the
value of POOL_CONFIG->"setting name" can be found in the properties of a pool, the value of
GLOBALS->"setting name" in the global BIG-IP DNS setting, and the value of path->"setting name"
These are measured values that come from path metrics. If there are no path metrics, the system
does not perform path metric calculations and computes the QoS score using the other
calculations. vs->"field" These are measured values that come from measurements the system makes
on virtual servers. If there are no measurements, the system does not perform these calculations
and computes the QoS score using the other calculations. Each QoS coefficient, its scale, default
value, upper limit, and whether a higher or lower value is more efficient are defined in the
table.
Coefficient | Scale | Default value | Upper limit | Is higher or lower value more efficient? |
---|---|---|---|---|
Round trip time (rtt) | Microseconds | 50 | 2,000,000 | L |
Completion rate (hit ratio) | Percentage of successfully transferred packets (0-100%) | 5 | 100% | H |
Hops | Number of intermediate systems transitions | 0 | 64 | L |
Packet rate | Packets per second | 1 | 700 | L |
bits/second | Bits per second throughput | 3 | 15000 | L |
Topology | Score that defines network proximity by comparing server and LDNS IP addresses (0-2 32 ) | 0 | 100 | H |
Virtual server capacity (vs capacity) | Number of nodes up | 0 | 20 | H |
Virtual server score (vs score) | User-defined ranking of virtual servers | 0 | 100 | H |
Link capacity (lcs) | Based on the target dynamic ratio | 30 | 2,000,000 | H |
About customizing the
QoS equation
When you customize the QoS equation, consider these three concepts:
- Scale
- The raw metrics for the coefficients in the QoS equation are on different scales. For example, completion rate is measured in percentages, while packet rate is measured in packets per second.
- Normalization
- BIG-IP DNS normalizes the raw metrics to values in the range of 0 - 10.
- Emphasis
- You can adjust coefficients to emphasize one normalized metric over another.
When you
customize the QoS equation configuration using the values in the table, if the completion rates
for two virtual servers are close, the system chooses the virtual server with the best packet
rate. If both the completion rates and the packet rates are close, the round trip time (RTT)
breaks the tie. In this example, BIG-IP DNS does not use the metrics for topology, hops, link
capacity, vs capacity, and kilobytes/second to determine how to distribute connections.
You can set a
value for either RTT or hops. If you set both, BIG-IP DNS incorporates the RTT and resets the
hops to 0 (zero).
Coefficient | Value |
---|---|
Round Trip Time | 50 |
Hops | 0 |
Topology | 0 |
Completion Rate | 5 |
Packet Rate | 10 |
VS Capacity | 0 |
Bits/second | 35 |
Link Capacity | 30 |
Virtual Server Score | 10 |
Kilobytes/Second (KBPS) | 3 |
Customizing the QoS equation for load balancing global traffic
Determine the pool to which you want to apply a customized QoS equation.
Customize the QoS equation to load balance the DNS name resolution requests the
members of this pool handle.
- On the Main tab, click.
- Click the name of the pool for which you want to modify the QoS equation.The Pool Properties screen displays.
- On the menu bar, clickMembers.The Members Properties screen displays.
- SelectQuality of Servicefrom either thePreferredorFallbacklist.The Quality of Service Weights area displays.
- Define the QoS coefficients for this pool.
- ClickUpdate.
About dynamic ratio
load balancing
When you use dynamic ratio load balancing, BIG-IP DNS treats dynamic load
balancing values as ratios, and distributes DNS name resolution requests to the virtual servers
in the pool in proportion to these ratios.
Consider a pool
named primaryOne, which contains two virtual servers: memberOne and memberTwo. primaryOne is
configured with the Preferred load balancing method set to
Round Trip Time
. BIG-IP DNS determines that the
round trip time for memberOne is 50 microseconds and the round trip time for memberTwo is 100
microseconds. When the Dynamic Ratio
setting on the primaryOne pool is disabled, BIG-IP DNS always sends DNS name resolution requests
to memberOne, because that virtual server has the lowest round trip time value. When the
Dynamic Ratio
setting on the
primaryOne pool is enabled, BIG-IP DNS treats the round trip time values as ratios and sends
twice as many DNS name resolution requests to memberOne as it sends to memberTwo, because the
round trip time for memberOne is twice as fast as the round trip time for memberTwo. Distributing DNS requests based on weighted virtual servers
Determine the pool to which you want to apply the dynamic ratio feature.
Configure BIG-IP DNS to use dynamic load balancing values as ratios, and distribute
DNS name resolution requests to virtual servers in a pool in proportion to these ratios.
- On the Main tab, click.The Pools list screen opens.
- Click the name of the pool that you want to modify.
- From theConfigurationlist, selectAdvanced.
- Select theDynamic Ratiocheck box.
- ClickUpdate.
Using the preferred load balancing method when metrics are unavailable
Configure BIG-IP DNS to use the preferred load balancing method assigned to a pool
even when metrics for the pool are unavailable. BIG-IP DNS uses old metrics, rather than
the alternate load balancing method assigned to the pool.
- On the Main tab, click.The Load Balancing configuration screen opens.
- Select theIgnore Path TTLcheck box.
- ClickUpdate.
BIG-IP DNS uses path information gathered during metrics collection even if the
time-to-live (TTL) value of that information has expired.
Configuring the
resources in a pool for manual resume
Determine the pool to which you want to apply the manual resume feature.
When a virtual server goes offline, BIG-IP DNS
proceeds to send DNS name resolution requests to other virtual servers, based on the
current load balancing method. By default, when the virtual server becomes available
again, BIG-IP DNS resumes sending requests to that resource. When you do not want BIG-IP
DNS to resume to send requests to the virtual servers in a pool immediately after the
resources become available, enable the manual resume feature on the pool.
- On the Main tab, click.The Pools list screen opens.
- Click the name of the pool that you want to modify.
- From theConfigurationlist, selectAdvanced.
- Select theManual Resumecheck box.
- ClickUpdate.
After a virtual server in this pool goes offline,
you must manually enable the virtual server before BIG-IP DNS can resume sending
requests to the virtual server.
Restoring availability of a pool member manually
Determine the virtual server that you want to manually enable.
When a virtual server in a pool that is configured for manual resume becomes
available, you must manually enable the virtual server before BIG-IP DNS can begin sending DNS
name resolution requests to the virtual server.
- On the Main tab, click.The Pools list screen opens.
- Click the name of the pool to which the virtual server you want to enable belongs.
- On the menu bar, clickMembers.
- Select the check box next to the virtual server that you want to enable, and then clickEnable.
The virtual server is now available to receive DNS name resolution
requests.
Best Practices: Dynamic load balancing and performance configuration
How do I ensure that the BIG-IP® DNS (formerly GTM)
has timely access to path and network traffic
metrics?
BIG-IP® DNS (formerly GTM)
has timely access to path and network traffic
metrics?Run a
big3d
agent on at least one BIG-IP® system in
each data center to ensure that the BIG-IP DNS
has timely access to path and network traffic metrics.How do I ensure that BIG-IP DNS can gather information for dynamic load balancing?
BIG-IP DNS
can gather information for dynamic load balancing?If you use dynamic load balancing modes, you must run a
big3d
agent on
every BIG-IP system in your network. How do I configure BIG-IP DNS for the best performance?
BIG-IP DNS
for the best performance?The load on the
big3d
agents depends on the timer settings that you assign
to the different types of data the agents collect. The shorter the timers, the more frequently
the agent needs to refresh the data. While short timers guarantee that you always have valid
data readily available for load balancing, they also increase the frequency of data collection. The more local DNS servers that make resolution requests, the more path data that the
big3d
agents have to collect. While round trip time for a given path may
vary constantly due to current network load, the number of hops along a network path between a
data center and a specific LDNS does not often change. Consequently, you may want to set short
timer settings for round trip time data so that it refreshes more often, but set high timer
settings for hops data because it does not need to be refreshed often. Troubleshooting
If you turn off the
big3d
agent on a BIG-IP system, the BIG-IP DNS
can no longer check the
availability of the server or its virtual servers. Therefore, the statistics screens display the
status of these servers as unknown.