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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
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.
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
