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Config Settings

See Block Device for additional details.

Generic IO Settings

rbd_compression_hint

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 data. If set to incompressible and the OSD compression setting is aggressive, the OSD will not attempt to compress data.

type

str

default

none

valid choices

• none

• compressible

• incompressible

rbd_read_from_replica_policy

Policy for determining which OSD will receive read operations. If set to default, each PG’s 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. Unlike rbd_balance_snap_reads and rbd_localize_snap_reads or rbd_balance_parent_reads and rbd_localize_parent_reads, it affects all read operations, not just snap or parent. Note: this feature requires the cluster to be configured with a minimum compatible OSD release of Octopus.

type

str

default

default

valid choices

• default

• balance

• localize

rbd_default_order

This configures the default object size for new images. The value is used as a power of two, meaning default_object_size = 2 ^ rbd_default_order. Configure a value between 12 and 25 (inclusive), translating to 4KiB lower and 32MiB upper limit.

type

uint

default

22

Cache Settings

Kernel Caching

The kernel driver for Ceph block devices can use the Linux page cache to improve performance.

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.

Option settings for RBD should be set in the [client] section of your configuration file or the central config store. These settings include:

rbd_cache

Enable caching for RADOS Block Device (RBD).

type

bool

default

true

rbd_cache_policy

Select the caching policy for librbd.

type

str

default

writearound

valid choices

• writethrough

• writeback

• writearound

rbd_cache_writethrough_until_flush

Start out in writethrough mode, and switch to writeback after the first flush request is received. Enabling is a conservative but safe strategy in case VMs running on RBD volumes are too old to send flushes, like the virtio driver in Linux kernels older than 2.6.32.

type

bool

default

true

rbd_cache_size

The per-volume RBD client cache size in bytes.

type

size

default

32Mi

policies

write-back and write-through

rbd_cache_max_dirty

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

type

size

default

24Mi

constraint

Must be less than rbd_cache_size.

policies

write-around and write-back

rbd_cache_target_dirty

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

type

size

default

16Mi

constraint

Must be less than rbd_cache_max_dirty.

policies

write-back

rbd_cache_max_dirty_age

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

type

float

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

number of sequential requests necessary to trigger readahead

type

uint

default

10

rbd_readahead_max_bytes

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

type

size

default

512Ki

rbd_readahead_disable_after_bytes

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

size

default

50Mi

Image Features

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

Layering

Description

Layering enables 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 acquire a lock on an object before making a write. Exclusive lock should only be enabled when a single client is accessing an image at any given 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, which means that they only store data that actually has been written, ie. they are sparse. Object map support helps track which objects actually exist (have data stored on a device). 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. It is also much faster to calculate the actual data usage of a snapshot or volume (rbd du).

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 enables rbd flatten to work on all 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 cannot be deleted until the snapshots are first deleted. Deep-flatten makes a parent independent of its clones, even if they have snapshots, at the expense of using additional OSD device space.

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 can utilize the journal to replicate a crash-consistent image to a remote cluster. It is best to let rbd-mirror manage this feature only as needed, as enabling it long term may result in substantial additional OSD space consumption.

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 in several ways. These all apply to a given image within a given process - the same image used in multiple places, e.g. two separate VMs, would have independent limits.

• IOPS: number of I/Os per second (any type of I/O)

• read IOPS: number of read I/Os per second

• write IOPS: number of write I/Os per second

• bps: bytes per second (any type of I/O)

• read bps: bytes per second read

• write bps: bytes per second written

Each of these limits operates independently of each other. They are all off by default. Every type of limit throttles I/O using a token bucket algorithm, with the ability to configure the limit (average speed over time) and potential for a higher rate (a burst) for a short period of time (burst_seconds). When any of these limits is reached, and there is no burst capacity left, librbd reduces the rate of that type of I/O to the limit.

For example, if a read bps limit of 100MB was configured, but writes were not limited, writes could proceed as quickly as possible, while reads would be throttled to 100MB/s on average. If a read bps burst of 150MB was set, and read burst seconds was set to five seconds, reads could proceed at 150MB/s for up to five seconds before dropping back to the 100MB/s limit.

The following options configure these throttles:

rbd_qos_iops_limit

the desired limit of IO operations per second

type

uint

default

0

rbd_qos_iops_burst

the desired burst limit of IO operations

type

uint

default

0

rbd_qos_iops_burst_seconds

the desired burst duration in seconds of IO operations

type

uint

default

1

min

1

rbd_qos_read_iops_limit

the desired limit of read operations per second

type

uint

default

0

rbd_qos_read_iops_burst

the desired burst limit of read operations

type

uint

default

0

rbd_qos_read_iops_burst_seconds

the desired burst duration in seconds of read operations

type

uint

default

1

min

1

rbd_qos_write_iops_limit

the desired limit of write operations per second

type

uint

default

0

rbd_qos_write_iops_burst

the desired burst limit of write operations

type

uint

default

0

rbd_qos_write_iops_burst_seconds

the desired burst duration in seconds of write operations

type

uint

default

1

min

1

rbd_qos_bps_limit

the desired limit of IO bytes per second

type

uint

default

0

rbd_qos_bps_burst

the desired burst limit of IO bytes

type

uint

default

0

rbd_qos_bps_burst_seconds

the desired burst duration in seconds of IO bytes

type

uint

default

1

min

1

rbd_qos_read_bps_limit

the desired limit of read bytes per second

type

uint

default

0

rbd_qos_read_bps_burst

the desired burst limit of read bytes

type

uint

default

0

rbd_qos_read_bps_burst_seconds

the desired burst duration in seconds of read bytes

type

uint

default

1

min

1

rbd_qos_write_bps_limit

the desired limit of write bytes per second

type

uint

default

0

rbd_qos_write_bps_burst

the desired burst limit of write bytes

type

uint

default

0

rbd_qos_write_bps_burst_seconds

the desired burst duration in seconds of write bytes

type

uint

default

1

min

1

rbd_qos_schedule_tick_min

This determines the minimum time (in milliseconds) at which I/Os can become unblocked if the limit of a throttle is hit. In terms of the token bucket algorithm, this is the minimum interval at which tokens are added to the bucket.

type

uint

default

50

min

1

rbd_qos_exclude_ops

Optionally exclude ops from QoS. This setting accepts either an integer bitmask value or comma-delimited string of op names. This setting is always internally stored as an integer bitmask value. The mapping between op bitmask value and op name is as follows: +1 -> read, +2 -> write, +4 -> discard, +8 -> write_same, +16 -> compare_and_write

type

str