Resource Limits

Nova supports configuring limits on individual resources including
CPU, memory, disk and network. These limits can be used to enforce basic
Quality-of-Service (QoS) policies on such resources.

Configuring resource limits

Resource quota enforcement support is specific to the virt driver in
use on compute hosts.

libvirt

The libvirt driver supports CPU, disk and VIF limits. Unfortunately
all of these work quite differently, as discussed below.

CPU limits

Libvirt enforces CPU limits in terms of shares and
quotas, configured via quota:cpu_shares and quota:cpu_period
/ quota:cpu_quota, respectively. Both are
implemented using the cgroups cpu
controller. Note that allowed values for shares are
platform dependant.

CPU shares are a proportional weighted share of total CPU resources
relative to other instances. It does not limit CPU usage if CPUs are not
busy. There is no unit and the value is purely relative to other
instances, so an instance configured with value of 2048 will get twice
as much CPU time as a VM configured with the value 1024. For example, to
configure a CPU share of 1024 for a flavor:

$ openstack flavor set $FLAVOR --property quota:cpu_shares=1024

The CPU quotas require both a period and quota. The CPU period
specifies the enforcement interval in microseconds, while the CPU quota
specifies the maximum allowed bandwidth in microseconds that the each
vCPU of the instance can consume. The CPU period must be in the range
1000 (1mS) to 1,000,000 (1s) or 0 (disabled). The CPU quota must be in
the range 1000 (1mS) to 2^64 or 0 (disabled). Where the CPU quota
exceeds the CPU period, this means the guest vCPU process is able to
consume multiple pCPUs worth of bandwidth. For example, to limit each
guest vCPU to 1 pCPU worth of runtime per period:

$ openstack flavor set $FLAVOR \
    --property quota:cpu_period=1000 \
    --property quota:cpu_quota=1000

To limit each guest vCPU to 2 pCPUs worth of runtime per period:

$ openstack flavor set $FLAVOR \
    --property quota:cpu_period=1000 \
    --property quota:cpu_quota=2000

Finally, to limit each guest vCPU to 0.5 pCPUs worth of runtime per
period:

$ openstack flavor set $FLAVOR \
    --property quota:cpu_period=1000 \
    --property quota:cpu_quota=500

CPU shares and CPU quotas can work hand-in-hand. For example, if two
instances were configured with quota:cpu_shares=1024 and quota:cpu_period=100000 (100mS) for both,
then configuring both with a quota:cpu_quota=75000 (75mS) will result
in them sharing a host CPU equally, with both getting exactly 50mS of
CPU time. If instead only one instance gets quota:cpu_quota=75000 (75mS) while the
other gets quota:cpu_quota=25000 (25mS), then the
first will get 3/4 of the time per period.

Memory Limits

The libvirt driver does not support memory limits.

Disk I/O Limits

Libvirt enforces disk limits through maximum disk read, write and
total bytes per second, using the quota:disk_read_bytes_sec, quota:disk_write_bytes_sec and quota:disk_total_bytes_sec extra specs,
respectively. It can also enforce disk limits through maximum disk read,
write and total I/O operations per second, using the quota:disk_read_iops_sec, quota:disk_write_iops_sec and quota:disk_total_iops_sec extra specs,
respectively. For example, to set a maximum disk write of 10 MB/sec for
a flavor:

$ openstack flavor set $FLAVOR \
    --property quota:disk_write_bytes_sec=10485760

Network bandwidth limits

Libvirt enforces network bandwidth limits through inbound and
outbound average, using the quota:vif_inbound_average and quota:vif_outbound_average extra specs,
respectively. In addition, optional peak values, which
specifies the maximum rate at which a bridge can send data (kB/s), and
burst values, which specifies the amount of bytes that can be
burst at peak speed (kilobytes), can be specified for both inbound and
outbound traffic, using the quota:vif_inbound_peak / quota:vif_outbound_peak and quota:vif_inbound_burst / quota:vif_outbound_burst extra specs,
respectively.

For example, to configure outbound traffic to an
average of 262 Mbit/s (32768 kB/s), a peak of 524 Mbit/s, and burst of
65536 kilobytes:

$ openstack flavor set $FLAVOR \
    --property quota:vif_outbound_average=32768 \
    --property quota:vif_outbound_peak=65536 \
    --property quota:vif_outbound_burst=65536

VMWare

In contrast to libvirt, the VMWare virt driver enforces resource
limits using consistent terminology, specifically through relative
allocation levels, hard upper limits and minimum reservations configured
via, for example, the quota:cpu_shares_level / quota:cpu_shares_share, quota:cpu_limit,
and quota:cpu_reservation extra specs,
respectively.

Allocation levels can be specified using one of high,
normal, low, or custom. When
custom is specified, the number of shares must be specified
using e.g. quota:cpu_shares_share. There is no unit
and the values are relative to other instances on the host. The upper
limits and reservations, by comparison, are measure in resource-specific
units, such as MHz for CPUs and will ensure that the instance never used
more than or gets less than the specified amount of the resource.

CPU limits

CPU limits are configured via the quota:cpu_shares_level / quota:cpu_shares_share, quota:cpu_limit,
and quota:cpu_reservation extra specs.

For example, to configure a CPU allocation level of
custom with 1024 shares:

$ openstack flavor set $FLAVOR \
    --quota:cpu_shares_level=custom \
    --quota:cpu_shares_share=1024

To configure a minimum CPU allocation of 1024 MHz and a maximum of
2048 MHz:

$ openstack flavor set $FLAVOR \
    --quota:cpu_reservation=1024 \
    --quota:cpu_limit=2048

Memory limits

Memory limits are configured via the quota:memory_shares_level / quota:memory_shares_share, quota:memory_limit, and quota:memory_reservation extra specs.

For example, to configure a memory allocation level of
custom with 1024 shares:

$ openstack flavor set $FLAVOR \
    --quota:memory_shares_level=custom \
    --quota:memory_shares_share=1024

To configure a minimum memory allocation of 1024 MB and a maximum of
2048 MB:

$ openstack flavor set $FLAVOR \
    --quota:memory_reservation=1024 \
    --quota:memory_limit=2048

Disk I/O limits

Disk I/O limits are configured via the quota:disk_io_shares_level / quota:disk_io_shares_share, quota:disk_io_limit, and quota:disk_io_reservation extra specs.

For example, to configure a disk I/O allocation level of
custom with 1024 shares:

$ openstack flavor set $FLAVOR \
    --quota:disk_io_shares_level=custom \
    --quota:disk_io_shares_share=1024

To configure a minimum disk I/O allocation of 1024 MB and a maximum
of 2048 MB:

$ openstack flavor set $FLAVOR \
    --quota:disk_io_reservation=1024 \
    --quota:disk_io_limit=2048

Network bandwidth limits

Network bandwidth limits are configured via the quota:vif_shares_level / quota:vif_shares_share, quota:vif_limit,
and quota:vif_reservation extra specs.

For example, to configure a network bandwidth allocation level of
custom with 1024 shares:

$ openstack flavor set $FLAVOR \
    --quota:vif_shares_level=custom \
    --quota:vif_shares_share=1024

To configure a minimum bandwidth allocation of 1024 Mbits/sec and a
maximum of 2048 Mbits/sec:

$ openstack flavor set $FLAVOR \
    --quota:vif_reservation=1024 \
    --quota:vif_limit=2048

Hyper-V

CPU limits

The Hyper-V driver does not support CPU limits.

Memory limits

The Hyper-V driver does not support memory limits.

Disk I/O limits

Hyper-V enforces disk limits through maximum total bytes and total
I/O operations per second, using the quota:disk_total_bytes_sec and quota:disk_total_iops_sec extra specs,
respectively. For example, to set a maximum disk read/write of 10 MB/sec
for a flavor:

$ openstack flavor set $FLAVOR \
    --property quota:disk_total_bytes_sec=10485760

Network bandwidth limits

The Hyper-V driver does not support network bandwidth limits.