Config Settings¶

See Block Device for additional details.

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: v0.70 (Emperor)
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: v0.70 (Emperor)
KRBD support: since v3.10
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

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 schedule tick min

Description: The minimum schedule tick (in milliseconds) for QoS.
Type: Unsigned Integer
Required: No
Default: 50