Config Settings

See Block Device for additional details.

Generic IO Settings

rbd compression hint

Description

Hint to send to the OSDs on write operations. If set to compressible and the OSD bluestore compression mode setting is passive, the OSD will attempt to compress the data. If set to incompressible and the OSD compression setting is aggressive, the OSD will not attempt to compress the data.

Type

Enum

Required

No

Default

none

Values

none, compressible, incompressible

rbd read from replica policy

Description

policy for determining which OSD will receive read operations. If set to default, the primary OSD will always be used for read operations. If set to balance, read operations will be sent to a randomly selected OSD within the replica set. If set to localize, read operations will be sent to the closest OSD as determined by the CRUSH map. Note: this feature requires the cluster to be configured with a minimum compatible OSD release of Octopus.

Type

Enum

Required

No

Default

default

Values

default, balance, localize

Cache Settings

The user space implementation of the Ceph block device (i.e., librbd) cannot take advantage of the Linux page cache, so it includes its own in-memory caching, called “RBD caching.” RBD caching behaves just like well-behaved hard disk caching. When the OS sends a barrier or a flush request, all dirty data is written to the OSDs. This means that using write-back caching is just as safe as using a well-behaved physical hard disk with a VM that properly sends flushes (i.e. Linux kernel >= 2.6.32). The cache uses a Least Recently Used (LRU) algorithm, and in write-back mode it can coalesce contiguous requests for better throughput.

The librbd cache is enabled by default and supports three different cache policies: write-around, write-back, and write-through. Writes return immediately under both the write-around and write-back policies, unless there are more than rbd cache max dirty unwritten bytes to the storage cluster. The write-around policy differs from the write-back policy in that it does not attempt to service read requests from the cache, unlike the write-back policy, and is therefore faster for high performance write workloads. Under the write-through policy, writes return only when the data is on disk on all replicas, but reads may come from the cache.

Prior to receiving a flush request, the cache behaves like a write-through cache to ensure safe operation for older operating systems that do not send flushes to ensure crash consistent behavior.

If the librbd cache is disabled, writes and reads go directly to the storage cluster, and writes return only when the data is on disk on all replicas.

Note

The cache is in memory on the client, and each RBD image has its own. Since the cache is local to the client, there’s no coherency if there are others accessing the image. Running GFS or OCFS on top of RBD will not work with caching enabled.

The ceph.conf file settings for RBD should be set in the [client] section of your configuration file. The settings include:

rbd cache

Description

Enable caching for RADOS Block Device (RBD).

Type

Boolean

Required

No

Default

true

rbd cache policy

Description

Select the caching policy for librbd.

Type

Enum

Required

No

Default

writearound

Values

writearound, writeback, writethrough

rbd cache writethrough until flush

Description

Start out in write-through mode, and switch to write-back after the first flush request is received. Enabling this is a conservative but safe setting in case VMs running on rbd are too old to send flushes, like the virtio driver in Linux before 2.6.32.

Type

Boolean

Required

No

Default

true

rbd cache size

Description

The RBD cache size in bytes.

Type

64-bit Integer

Required

No

Default

32 MiB

Policies

write-back and write-through

rbd cache max dirty

Description

The dirty limit in bytes at which the cache triggers write-back. If 0, uses write-through caching.

Type

64-bit Integer

Required

No

Constraint

Must be less than rbd cache size.

Default

24 MiB

Policies

write-around and write-back

rbd cache target dirty

Description

The dirty target before the cache begins writing data to the data storage. Does not block writes to the cache.

Type

64-bit Integer

Required

No

Constraint

Must be less than rbd cache max dirty.

Default

16 MiB

Policies

write-back

rbd cache max dirty age

Description

The number of seconds dirty data is in the cache before writeback starts.

Type

Float

Required

No

Default

1.0

Policies

write-back

Read-ahead Settings

librbd supports read-ahead/prefetching to optimize small, sequential reads. This should normally be handled by the guest OS in the case of a VM, but boot loaders may not issue efficient reads. Read-ahead is automatically disabled if caching is disabled or if the policy is write-around.

rbd readahead trigger requests

Description

Number of sequential read requests necessary to trigger read-ahead.

Type

Integer

Required

No

Default

10

rbd readahead max bytes

Description

Maximum size of a read-ahead request. If zero, read-ahead is disabled.

Type

64-bit Integer

Required

No

Default

512 KiB

rbd readahead disable after bytes

Description

After this many bytes have been read from an RBD image, read-ahead is disabled for that image until it is closed. This allows the guest OS to take over read-ahead once it is booted. If zero, read-ahead stays enabled.

Type

64-bit Integer

Required

No

Default

50 MiB

Image Features

RBD supports advanced features which can be specified via the command line when creating images or the default features can be specified via Ceph config file via ‘rbd_default_features = <sum of feature numeric values>’ or ‘rbd_default_features = <comma-delimited list of CLI values>’

Layering

Description

Layering enables you to use cloning.

Internal value

1

CLI value

layering

Added in

v0.52 (Bobtail)

KRBD support

since v3.10

Default

yes

Striping v2

Description

Striping spreads data across multiple objects. Striping helps with parallelism for sequential read/write workloads.

Internal value

2

CLI value

striping

Added in

v0.55 (Bobtail)

KRBD support

since v3.10 (default striping only, “fancy” striping added in v4.17)

Default

yes

Exclusive locking

Description

When enabled, it requires a client to get a lock on an object before making a write. Exclusive lock should only be enabled when a single client is accessing an image at the same time.

Internal value

4

CLI value

exclusive-lock

Added in

v0.92 (Hammer)

KRBD support

since v4.9

Default

yes

Object map

Description

Object map support depends on exclusive lock support. Block devices are thin provisioned—meaning, they only store data that actually exists. Object map support helps track which objects actually exist (have data stored on a drive). Enabling object map support speeds up I/O operations for cloning; importing and exporting a sparsely populated image; and deleting.

Internal value

8

CLI value

object-map

Added in

v0.93 (Hammer)

KRBD support

since v5.3

Default

yes

Fast-diff

Description

Fast-diff support depends on object map support and exclusive lock support. It adds another property to the object map, which makes it much faster to generate diffs between snapshots of an image, and the actual data usage of a snapshot much faster.

Internal value

16

CLI value

fast-diff

Added in

v9.0.1 (Infernalis)

KRBD support

since v5.3

Default

yes

Deep-flatten

Description

Deep-flatten makes rbd flatten work on all the snapshots of an image, in addition to the image itself. Without it, snapshots of an image will still rely on the parent, so the parent will not be delete-able until the snapshots are deleted. Deep-flatten makes a parent independent of its clones, even if they have snapshots.

Internal value

32

CLI value

deep-flatten

Added in

v9.0.2 (Infernalis)

KRBD support

since v5.1

Default

yes

Journaling

Description

Journaling support depends on exclusive lock support. Journaling records all modifications to an image in the order they occur. RBD mirroring utilizes the journal to replicate a crash consistent image to a remote cluster.

Internal value

64

CLI value

journaling

Added in

v10.0.1 (Jewel)

KRBD support

no

Default

no

Data pool

Description

On erasure-coded pools, the image data block objects need to be stored on a separate pool from the image metadata.

Internal value

128

Added in

v11.1.0 (Kraken)

KRBD support

since v4.11

Default

no

Operations

Description

Used to restrict older clients from performing certain maintenance operations against an image (e.g. clone, snap create).

Internal value

256

Added in

v13.0.2 (Mimic)

KRBD support

since v4.16

Migrating

Description

Used to restrict older clients from opening an image when it is in migration state.

Internal value

512

Added in

v14.0.1 (Nautilus)

KRBD support

no

Non-primary

Description

Used to restrict changes to non-primary images using snapshot-based mirroring.

Internal value

1024

Added in

v15.2.0 (Octopus)

KRBD support

no

QOS Settings

librbd supports limiting per image IO, controlled by the following settings.

rbd qos iops limit

Description

The desired limit of IO operations per second.

Type

Unsigned Integer

Required

No

Default

0

rbd qos bps limit

Description

The desired limit of IO bytes per second.

Type

Unsigned Integer

Required

No

Default

0

rbd qos read iops limit

Description

The desired limit of read operations per second.

Type

Unsigned Integer

Required

No

Default

0

rbd qos write iops limit

Description

The desired limit of write operations per second.

Type

Unsigned Integer

Required

No

Default

0

rbd qos read bps limit

Description

The desired limit of read bytes per second.

Type

Unsigned Integer

Required

No

Default

0

rbd qos write bps limit

Description

The desired limit of write bytes per second.

Type

Unsigned Integer

Required

No

Default

0

rbd qos iops burst

Description

The desired burst limit of IO operations.

Type

Unsigned Integer

Required

No

Default

0

rbd qos bps burst

Description

The desired burst limit of IO bytes.

Type

Unsigned Integer

Required

No

Default

0

rbd qos read iops burst

Description

The desired burst limit of read operations.

Type

Unsigned Integer

Required

No

Default

0

rbd qos write iops burst

Description

The desired burst limit of write operations.

Type

Unsigned Integer

Required

No

Default

0

rbd qos read bps burst

Description

The desired burst limit of read bytes.

Type

Unsigned Integer

Required

No

Default

0

rbd qos write bps burst

Description

The desired burst limit of write bytes.

Type

Unsigned Integer

Required

No

Default

0

rbd qos iops burst seconds

Description

The desired burst duration in seconds of IO operations.

Type

Unsigned Integer

Required

No

Default

1

rbd qos bps burst seconds

Description

The desired burst duration in seconds of IO bytes.

Type

Unsigned Integer

Required

No

Default

1

rbd qos read iops burst seconds

Description

The desired burst duration in seconds of read operations.

Type

Unsigned Integer

Required

No

Default

1

rbd qos write iops burst seconds

Description

The desired burst duration in seconds of write operations.

Type

Unsigned Integer

Required

No

Default

1

rbd qos read bps burst seconds

Description

The desired burst duration in seconds of read bytes.

Type

Unsigned Integer

Required

No

Default

1

rbd qos write bps burst seconds

Description

The desired burst duration in seconds of write bytes.

Type

Unsigned Integer

Required

No

Default

1

rbd qos schedule tick min

Description

The minimum schedule tick (in milliseconds) for QoS.

Type

Unsigned Integer

Required

No

Default

50