Storage Quality of Service

Overview

Storage QoS (Quality of Service) specifications define performance boundaries for block storage volumes — capping IOPS, throughput, and burst behavior at the hypervisor-to-storage layer. QoS specs are attached to volume types, so every volume created from that type automatically inherits the performance constraints. This prevents high-throughput workloads from starving adjacent volumes on shared storage.

Administrator Access Required — This operation requires the admin role. Contact your Polystack administrator if you do not have sufficient permissions.

Prerequisites

Admin credentials sourced from openrc.sh
Block Storage service deployed and healthy
Volume types configured (Volume Types Admin Guide)
QoS support enabled on the target storage backend (Ceph, NetApp, or Pure Storage)

QoS Specification Parameters

Storage QoS specs define performance limits using key-value pairs. The specific keys supported depend on the storage backend driver.

Standard QoS Keys (all backends)

Key	Unit	Description
`total_iops_sec`	IOPS	Maximum combined read + write IOPS
`read_iops_sec`	IOPS	Maximum read IOPS
`write_iops_sec`	IOPS	Maximum write IOPS
`total_bytes_sec`	bytes/s	Maximum combined read + write throughput
`read_bytes_sec`	bytes/s	Maximum read throughput
`write_bytes_sec`	bytes/s	Maximum write throughput
`total_iops_sec_max`	IOPS	Burst IOPS ceiling
`total_bytes_sec_max`	bytes/s	Burst throughput ceiling
`size_iops_sec`	IOPS/GB	IOPS scaling factor (per GB of volume size)

size_iops_sec scales limits proportionally to volume size. A spec of size_iops_sec=10 grants a 100 GB volume 1,000 IOPS and a 500 GB volume 5,000 IOPS automatically.

Ceph RBD QoS Keys

Ceph RBD enforces QoS through its rbd_qos_* parameters, configured in ceph.conf and overridable per volume via extra specs.

Key	Unit	Description
`rbd_qos_iops_limit`	IOPS	Combined read+write IOPS cap
`rbd_qos_read_iops_limit`	IOPS	Read IOPS cap
`rbd_qos_write_iops_limit`	IOPS	Write IOPS cap
`rbd_qos_bps_limit`	bytes/s	Combined throughput cap
`rbd_qos_read_bps_limit`	bytes/s	Read throughput cap
`rbd_qos_write_bps_limit`	bytes/s	Write throughput cap
`rbd_qos_iops_burst`	IOPS	Burst IOPS above the sustained limit
`rbd_qos_bps_burst`	bytes/s	Burst throughput above sustained limit

Ceph QoS is enforced at the librbd layer — inside the QEMU process on the hypervisor. It does not require OSD-level configuration changes and applies per-volume independently.

NetApp ONTAP QoS Keys

Key	Unit	Description
`netapp:qos_policy_group`	Policy name	Reference to an ONTAP QoS policy group
`netapp:qos_adaptive_policy_group`	Policy name	Reference to an ONTAP adaptive QoS policy
`total_iops_sec`	IOPS	Enforced via ONTAP QoS policy
`total_bytes_sec`	bytes/s	Enforced via ONTAP QoS policy

NetApp adaptive QoS adjusts limits automatically based on volume size (TB) — expected and peak IOPS/TB values are set in the ONTAP adaptive policy group.

Pure Storage QoS Keys

Key	Unit	Description
`pure_qos_max_bandwidth`	bytes/s	Maximum combined read + write throughput
`pure_qos_max_iops`	IOPS	Maximum combined IOPS

Pure Storage enforces QoS at the array level — limits apply to the volume regardless of which host or hypervisor is accessing it.

Create QoS Specifications

Dashboard
CLI

Navigate to QoS Specs

Navigate to Storage > Volume Types (QoS Specs tab, admin view). Click Create QoS Spec.

Define the spec

Field	Value	Description
Name	e.g., `bronze-1000iops`	Descriptive name
Consumer	`front-end`	Where limits are enforced: `front-end` (hypervisor), `back-end` (storage array), or `both`

Click Confirm to save the spec.

Add key-value pairs

Open the newly created spec and click Edit Keys. Add the desired performance parameters:

Key	Example Value
`total_iops_sec`	`1000`
`total_bytes_sec`	`104857600` (100 MB/s)
`total_iops_sec_max`	`2000`

Click Confirm to apply the key-value pairs to the spec.

Associate with a volume type

Navigate to Storage > Volume Types (admin view), select a volume type, click Manage QoS Spec, and select the QoS spec from the dropdown.

All new volumes created from this volume type inherit the QoS limits automatically.

Authenticate

Source your credentials file to authenticate with the Polystack platform:

Load credentials

source openrc.sh

Your administrator provides the RC (credentials) file for your project. See CLI Setup for configuration details.

Create the QoS spec

Create bronze QoS spec (1,000 IOPS, 100 MB/s)

openstack volume qos create bronze-1000iops \
  --consumer front-end \
  --property total_iops_sec=1000 \
  --property total_bytes_sec=104857600 \
  --property total_iops_sec_max=2000

Create gold QoS spec (10,000 IOPS, 1 GB/s)

openstack volume qos create gold-10000iops \
  --consumer front-end \
  --property total_iops_sec=10000 \
  --property total_bytes_sec=1073741824 \
  --property total_iops_sec_max=15000 \
  --property total_bytes_sec_max=2147483648

The --consumer flag controls enforcement location:

front-end — enforced by the hypervisor (libvirt QoS)
back-end — enforced by the storage array
both — enforced at both layers

Associate with a volume type

Associate QoS spec with a volume type

openstack volume qos associate bronze-1000iops standard-ssd

Associate gold spec with premium volume type

openstack volume qos associate gold-10000iops performance-ssd

Verify the association

Show volume type with QoS association

openstack volume type show standard-ssd -c qos_specs_id

List all QoS specs

openstack volume qos list

The volume type displays the associated QoS spec ID. New volumes created from this type are QoS-limited automatically.

Backend-Specific QoS Examples

Ceph RBD
Standard (libvirt)
Size-Scaled QoS

For Ceph RBD backends, use the rbd_qos_* keys. These are enforced by the QEMU librbd driver on the hypervisor.

Create Ceph-specific QoS spec

openstack volume qos create ceph-standard-qos \
  --consumer front-end \
  --property rbd_qos_iops_limit=1000 \
  --property rbd_qos_bps_limit=104857600 \
  --property rbd_qos_iops_burst=2000 \
  --property rbd_qos_bps_burst=209715200

Associate with Ceph volume type

openstack volume qos associate ceph-standard-qos ceph-ssd

Set rbd_qos_iops_burst to 2× the sustained rbd_qos_iops_limit to absorb short IO spikes (e.g., application startup, database checkpoint) without violating the sustained limit.

For standard backends where QoS is enforced at the hypervisor by libvirt:

Tiered QoS specs — bronze, silver, gold

# Bronze: 500 IOPS, 50 MB/s
openstack volume qos create bronze \
  --consumer front-end \
  --property total_iops_sec=500 \
  --property total_bytes_sec=52428800

# Silver: 2,000 IOPS, 200 MB/s
openstack volume qos create silver \
  --consumer front-end \
  --property total_iops_sec=2000 \
  --property total_bytes_sec=209715200

# Gold: 10,000 IOPS, 1 GB/s with burst
openstack volume qos create gold \
  --consumer front-end \
  --property total_iops_sec=10000 \
  --property total_bytes_sec=1073741824 \
  --property total_iops_sec_max=15000 \
  --property total_bytes_sec_max=2147483648

Use size_iops_sec to scale IOPS proportionally with volume size — larger volumes receive more IOPS automatically.

Create size-scaled QoS spec (10 IOPS per GB)

openstack volume qos create scaled-10iops-per-gb \
  --consumer front-end \
  --property size_iops_sec=10

Effective IOPS by volume size with this spec:

100 GB volume → 1,000 IOPS
500 GB volume → 5,000 IOPS
2,000 GB volume → 20,000 IOPS

Size-scaled QoS is ideal for database volume types where larger databases justifiably require more IO capacity, and smaller test databases should receive proportionally less.

QoS Capability Comparison

Capability	Ceph RBD	NetApp ONTAP	Pure Storage	Standard (libvirt)
IOPS limit	Yes	Yes	Yes	Yes
Throughput limit	Yes	Yes	Yes	Yes
Burst IOPS	Yes	Yes (adaptive)	No	Yes
Burst throughput	Yes	Yes (adaptive)	No	Yes
Per-read/write limits	Yes	No	No	Yes
Size-scaled IOPS	No	Yes (adaptive)	No	Yes
Enforcement layer	Hypervisor (librbd)	Array	Array	Hypervisor (libvirt)
Live update	Yes (libvirt)	Yes	Yes	Yes (libvirt)

Manage QoS Specs

List all QoS specifications

openstack volume qos list

Show a QoS spec and its properties

openstack volume qos show bronze-1000iops

Update a QoS spec property

openstack volume qos set bronze-1000iops \
  --property total_iops_sec=1500

Remove a property from a QoS spec

openstack volume qos unset bronze-1000iops --property total_iops_sec_max

Disassociate QoS spec from a volume type

openstack volume qos disassociate bronze-1000iops standard-ssd

Delete a QoS spec

openstack volume qos delete bronze-1000iops

Deleting a QoS spec that is still associated with a volume type will fail. Disassociate from all volume types before deleting the spec.

Monitoring QoS Compliance

Polystack Dashboard (Monitoring)
CLI

Navigate to Monitoring → Storage → Volume Metrics in Polystack Monitoring. Per-volume IOPS and throughput graphs show whether volumes are hitting their QoS limits:

IOPS at or near the total_iops_sec limit — the volume is QoS-constrained
Sustained IOPS below limit — the workload does not require the full allocation
Burst IOPS exceeding sustained limit but below burst cap — normal burst behavior

Set Monitoring alerts for volumes that sustain QoS-constrained behavior for more than 5 minutes — this signals that the volume type’s QoS limit may be too low for the workload.

Show volume details including QoS association

openstack volume show <volume-id> -c volume_type -c metadata

Show current QoS spec for a volume type

openstack volume type show <volume-type> -c qos_specs_id

For Ceph-backed volumes, verify QoS enforcement on the hypervisor:

Check RBD QoS settings on running volume (on compute node)

virsh domblkstat <domain-name> <disk-device>

Best Practices

Align with Volume Types

Define one QoS spec per volume tier (bronze, silver, gold). Attach specs to volume types so QoS is automatically applied without per-volume manual steps.

Set Burst at 2x Sustained

Configure burst limits at 2× the sustained limit to absorb application startup and checkpoint IO spikes without permanently raising the sustained cap.

Use Size-Scaled for Databases

Apply size_iops_sec to database volume types. Larger database volumes automatically receive proportionally more IOPS without manual reconfiguration.

Monitor Before Limiting

Baseline actual workload IOPS using Monitoring before applying QoS limits. Setting limits below actual peak workload demand causes application performance degradation.

Tiered QoS Profiles

Polystack provides pre-defined tiered QoS profiles that align with common workload classes. Each profile sets per-instance IOPS limits, bandwidth caps, and burst allowances with separate read and write thresholds.

Profile	IOPS (sustained)	Throughput	Burst IOPS	Target Workload
Bronze	500	50 MB/s	1,000	Development, test environments
Silver	2,000	200 MB/s	4,000	General-purpose applications
Gold	10,000	1 GB/s	15,000	Transactional databases, analytics
Platinum	50,000	3 GB/s	75,000	High-frequency trading, real-time ingestion

Key capabilities of tiered QoS profiles:

Per-instance enforcement — limits apply at the individual instance level, not shared across a tenant
Read/write-differentiated limits — separate IOPS and bandwidth caps for read and write operations allow tuning for read-heavy or write-heavy workloads independently
Burst support — short IO spikes (application startup, checkpoints) are absorbed up to the burst ceiling without throttling

Assign profiles to volume types so every volume inherits the correct QoS automatically. See Storage Tiers for aligning QoS profiles with hardware tiers.

Next Steps

Volume Types Admin

Create and manage volume types to associate with QoS specs

Storage Tiers

Configure tiered storage pools aligned with QoS tiers

Storage Backends

Review backend-specific capabilities and QoS driver support

Block Storage Architecture

Understand how QoS is enforced across the storage service stack

Core Services

Other Services

Overview

QoS Specification Parameters

Create QoS Specifications

Navigate to QoS Specs

Define the spec

Add key-value pairs

Associate with a volume type

Authenticate

Create the QoS spec

Associate with a volume type

Verify the association

Backend-Specific QoS Examples

QoS Capability Comparison

Manage QoS Specs

Monitoring QoS Compliance

Best Practices

Align with Volume Types

Set Burst at 2x Sustained

Use Size-Scaled for Databases

Monitor Before Limiting

Tiered QoS Profiles

Next Steps

Volume Types Admin

Storage Tiers

Storage Backends

Block Storage Architecture

​Overview

​QoS Specification Parameters

​Create QoS Specifications

Navigate to QoS Specs

Define the spec

Add key-value pairs

Associate with a volume type

Authenticate

Create the QoS spec

Associate with a volume type

Verify the association

​Backend-Specific QoS Examples

​QoS Capability Comparison

​Manage QoS Specs

​Monitoring QoS Compliance

​Best Practices

Align with Volume Types

Set Burst at 2x Sustained

Use Size-Scaled for Databases

Monitor Before Limiting

​Tiered QoS Profiles

​Next Steps

Volume Types Admin

Storage Tiers

Storage Backends

Block Storage Architecture

Overview

QoS Specification Parameters

Create QoS Specifications

Backend-Specific QoS Examples

QoS Capability Comparison

Manage QoS Specs

Monitoring QoS Compliance

Best Practices

Tiered QoS Profiles

Next Steps