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Chapter 6. OSD Configuration Reference

You can configure Ceph OSDs in the Ceph configuration file, but Ceph OSDs can use the default values and a very minimal configuration. A minimal Ceph OSD configuration sets the osd journal size and osd host options, and uses default values for almost everything else.

Ceph OSDs are numerically identified in incremental fashion, beginning with 0 using the following convention: 

osd.0
osd.1
osd.2

In a configuration file, you can specify settings for all Ceph OSDs in the cluster by adding configuration settings to the [osd] section of the configuration file. To add settings directly to a particular Ceph OSD (for example, osd host), enter it in a section specific only to that OSD in the Ceph configuration file. For example: 

[osd]
osd journal size = 1024

[osd.0]
osd host = osd-host-a

[osd.1]
osd host = osd-host-b

6.1. General Settings

The following settings provide a Ceph OSD’s ID, and determine paths to data and journals. Ceph deployment scripts typically generate the UUID automatically.

Important

Red Hat does not recommend changing the default paths for data or journals, as it makes it more problematic to troubleshoot Ceph later.

The journal size should be at least twice the product of the expected drive speed multiplied by the value of the filestore max sync interval option. However, the most common practice is to partition the journal drive (often an SSD), and mount it such that Ceph uses the entire partition for the journal.

osd uuid
Description
The universally unique identifier (UUID) for the Ceph OSD.
Type
UUID
Default
The UUID.
Note
The osd uuid applies to a single Ceph OSD. The fsid applies to the entire cluster.
osd data
Description
The path to the OSD’s data. You must create the directory when deploying Ceph. Mount a drive for OSD data at this mount point. Red Hat does not recommend changing the default.
Type
String
Default
/var/lib/ceph/osd/$cluster-$id
osd max write size
Description
The maximum size of a write in megabytes.
Type
32-bit Integer
Default
90
osd client message size cap
Description
The largest client data message allowed in memory.
Type
64-bit Integer Unsigned
Default
500MB default. 500*1024L*1024L
osd class dir
Description
The class path for RADOS class plug-ins.
Type
String
Default
$libdir/rados-classes

6.2. Journal Settings

By default, Ceph expects that you will store a Ceph OSD’s journal with the following path: 

/var/lib/ceph/osd/$cluster-$id/journal

Without performance optimization, Ceph stores the journal on the same disk as the Ceph OSD’s data. A Ceph OSD optimized for performance can use a separate disk to store journal data, for example, a solid state drive delivers high performance journaling.

A journal size should find the product of the filestore max sync interval and the expected throughput, and multiply the product by two (2): 

osd journal size = <2 * (expected throughput * filestore max sync interval)>

The expected throughput number should include the expected disk throughput (that is, sustained data transfer rate), and network throughput. For example, a 7200 RPM disk will likely have approximately 100 MB/s. Taking the min() of the disk and network throughput should provide a reasonable expected throughput. Some users just start off with a 10GB journal size. For example: 

osd journal size = 10000
Warning

Sizing the journal correctly for your OSDs is important. Using a small journal will lead to a slower recovery in the event of an OSD failure. The number of recovery threads has to be decreased in order to have a stable recovery by keeping pressure in journal at an acceptable level. Also, committing transactions to the file store will be slower and could lead to the file store hanging if the queued transaction size is bigger than the journal size.

osd journal
Description
The path to the OSD’s journal. This may be a path to a file or a block device (such as a partition of an SSD). If it is a file, you must create the directory to contain it. We recommend using a drive separate from the osd data drive.
Type
String
Default
/var/lib/ceph/osd/$cluster-$id/journal
osd journal size
Description
The size of the journal in megabytes. If this is 0, and the journal is a block device, the entire block device is used. This is ignored if the journal is a block device, and the entire block device is used.
Type
32-bit Integer
Default
5120
Recommended
Begin with 1GB. Should be at least twice the product of the expected speed multiplied by filestore max sync interval.

6.3. Scrubbing

In addition to making multiple copies of objects, Ceph insures data integrity by scrubbing placement groups. Ceph scrubbing is analogous to the fsck command on the object storage layer.

For each placement group, Ceph generates a catalog of all objects and compares each primary object and its replicas to ensure that no objects are missing or mismatched.

Light scrubbing (daily) checks the object size and attributes. Deep scrubbing (weekly) reads the data and uses checksums to ensure data integrity.

Scrubbing is important for maintaining data integrity, but it can reduce performance. Adjust the following settings to increase or decrease scrubbing operations.

osd max scrubs
Description
The maximum number of simultaneous scrub operations for a Ceph OSD.
Type
32-bit Int
Default
1
osd scrub thread timeout
Description
The maximum time in seconds before timing out a scrub thread.
Type
32-bit Integer
Default
60
osd scrub finalize thread timeout
Description
The maximum time in seconds before timing out a scrub finalize thread.
Type
32-bit Integer
Default
60*10
osd scrub begin hour
Description
The earliest hour that light or deep scrubbing can begin. It is used with the osd scrub end hour parameter to define a scrubbing time window and allows constraining scrubbing to off-peak hours. The setting takes an integer to specify the hour on the 24-hour cycle where 0 represents the hour from 12:01 a.m. to 1:00 a.m., 13 represents the hour from 1:01 p.m. to 2:00 p.m., and so on.
Type
32-bit Integer
Default
0 for 12:01 to 1:00 a.m.
osd scrub end hour
Description
The latest hour that light or deep scrubbing can begin. It is used with the osd scrub begin hour parameter to define a scrubbing time window and allows constraining scrubbing to off-peak hours. The setting takes an integer to specify the hour on the 24-hour cycle where 0 represents the hour from 12:01 a.m. to 1:00 a.m., 13 represents the hour from 1:01 p.m. to 2:00 p.m., and so on. The end hour must be greater than the begin hour.
Type
32-bit Integer
Default
24 for 11:01 p.m. to 12:00 a.m.
osd scrub load threshold
Description
The maximum load. Ceph will not scrub when the system load (as defined by the getloadavg() function) is higher than this number. Default is 0.5.
Type
Float
Default
0.5
osd scrub min interval
Description
The maximum interval in seconds for scrubbing the Ceph OSD when the Red Hat Ceph Storage cluster load is low.
Type
Float
Default
Once per day. 60*60*24
osd scrub max interval
Description
The maximum interval in seconds for scrubbing the Ceph OSD irrespective of cluster load.
Type
Float
Default
Once per week. 7*60*60*24
osd scrub interval randomize ratio
Description
Takes the ration and randomizes the scheduled scrub between osd scrub min interval and osd scrub max interval.
Type
Float
Default
0.5.
osd scrub chunk min
Description
The object store is partitioned into chunks which end on hash boundaries. For chunky scrubs, Ceph scrubs objects one chunk at a time with writes blocked for that chunk. The osd scrub chunk min setting represents minimum number of chunks to scrub.
Type
32-bit Integer
Default
5
osd scrub chunk max
Description
The maximum number of chunks to scrub.
Type
32-bit Integer
Default
25
osd scrub sleep
Description
The time to sleep between deep scrub operations.
Type
Float
Default
0 (or off).
osd deep scrub interval
Description
The interval for deep scrubbing, that is fully reading all data). The osd scrub load threshold parameter does not affect this setting.
Type
Float
Default
Once per week. 60*60*24*7
osd deep scrub stride
Description
Read size when doing a deep scrub.
Type
32-bit Integer
Default
512 KB. 524288

6.4. Operations

Operations settings allow you to configure the number of threads for servicing requests. If you set the osd op threads parameter to 0, it disables multi-threading.

By default, Ceph uses two threads with a 30 second timeout and a 30 second complaint time if an operation does not complete within those time parameters. Set operations priority weights between client operations and recovery operations to ensure optimal performance during recovery.

osd op threads
Description
The number of threads to service Ceph OSD operations. Set to 0 to disable it. Increasing the number might increase the request processing rate.
Type
32-bit Integer
Default
2
osd client op priority
Description
The priority set for client operations. It is relative to osd recovery op priority.
Type
32-bit Integer
Default
63
Valid Range
1-63
osd recovery op priority
Description
The priority set for recovery operations. It is relative to osd client op priority.
Type
32-bit Integer
Default
10
Valid Range
1-63
osd op thread timeout
Description
The Ceph OSD operation thread timeout in seconds.
Type
32-bit Integer
Default
30
osd op complaint time
Description
An operation becomes complaint worthy after the specified number of seconds have elapsed.
Type
Float
Default
30
osd disk threads
Description
The number of disk threads, which are used to perform background disk intensive OSD operations such as scrubbing and snap trimming.
Type
32-bit Integer
Default
1
osd disk thread ioprio class
Description

Sets the ioprio_set(2) I/O scheduling class for the disk thread. Acceptable values are:

  • idle
  • be

  • rt

    The idle class means the disk thread will have lower priority than any other thread in the OSD. This is useful to slow down scrubbing on an OSD that is busy handling client operations.

    The be class is the default and is the same priority as all other threads in the OSD.

    The rt class means the disk thread will have precedence over all other threads in the OSD. This is useful if scrubbing is much needed and must make progress at the expense of client operations.

Type
String
Default
an empty string
osd disk thread ioprio priority
Description
It sets the ioprio_set(2) I/O scheduling priority of the disk thread ranging from 0 (highest) to 7 (lowest). If all OSDs on a given host were in class idle and compete for I/O due to controller congestion, it can be used to lower the disk thread priority of one OSD to 7 so that another OSD with priority 0 can potentially scrub faster.
Type
Integer in the range of 0 to 7 or -1 if not to be used.
Default
-1
Important

The osd disk thread ioprio class and osd disk thread ioprio priority options will only be used if both are set to a non default value. In addition, it only works with the Linux Kernel CFQ scheduler.

osd op history size
Description
The maximum number of completed operations to track.
Type
32-bit Unsigned Integer
Default
20
osd op history duration
Description
The oldest completed operation to track.
Type
32-bit Unsigned Integer
Default
600
osd op log threshold
Description
How many operations logs to display at once.
Type
32-bit Integer
Default
5
osd op timeout
Description
The time in seconds after which running OSD operations time out.
Type
Integer
Default
0
Important

Do not set the osd op timeout option unless your clients can handle the consequences. For example, setting this parameter on clients running in virtual machines can lead to data corruption because the virtual machines interpret this timeout as a hardware failure.

6.5. Backfilling

When you add Ceph OSDs to a cluster or remove them from the cluster, the CRUSH algorithm rebalances the cluster by moving placement groups to or from Ceph OSDs to restore the balance. The process of migrating placement groups and the objects they contain can reduce the cluster operational performance considerably. To maintain operational performance, Ceph performs this migration with the 'backfill' process, which allows Ceph to set backfill operations to a lower priority than requests to read or write data.

osd max backfills
Description
The maximum number of backfill operations allowed to or from a single OSD.
Type
64-bit Unsigned Integer
Default
10
osd backfill scan min
Description
The minimum number of objects per backfill scan.
Type
32-bit Integer
Default
64
osd backfill scan max
Description
The maximum number of objects per backfill scan.
Type
32-bit Integer
Default
512
osd backfill full ratio
Description
Refuse to accept backfill requests when the Ceph OSD’s full ratio is above this value.
Type
Float
Default
0.85
osd backfill retry interval
Description
The number of seconds to wait before retrying backfill requests.
Type
Double
Default
10.0

6.6. OSD Map

OSD maps reflect the OSD daemons operating in the cluster. Over time, the number of map epochs increases. Ceph provides the following settings to ensure that Ceph performs well as the OSD map grows larger.

osd map dedup
Description
Enable removing duplicates in the OSD map.
Type
Boolean
Default
true
osd map cache size
Description
The size of the OSD map cache in megabytes.
Type
32-bit Integer
Default
500
osd map cache bl size
Description
The size of the in-memory OSD map cache in OSD daemons.
Type
32-bit Integer
Default
50
osd map cache bl inc size
Description
The size of the in-memory OSD map cache incrementals in OSD daemons.
Type
32-bit Integer
Default
100
osd map message max
Description
The maximum map entries allowed per MOSDMap message.
Type
32-bit Integer
Default
100

6.7. Recovery

When the cluster starts or when a Ceph OSD terminates unexpectedly and restarts, the OSD begins peering with other Ceph OSDs before write operation can occur.

If a Ceph OSD crashes and comes back online, usually it will be out of sync with other Ceph OSDs containing more recent versions of objects in the placement groups. When this happens, the Ceph OSD goes into recovery mode and seeks to get the latest copy of the data and bring its map back up to date. Depending upon how long the Ceph OSD was down, the OSD’s objects and placement groups may be significantly out of date. Also, if a failure domain went down (for example, a rack), more than one Ceph OSD may come back online at the same time. This can make the recovery process time consuming and resource intensive.

To maintain operational performance, Ceph performs recovery with limitations on the number recovery requests, threads and object chunk sizes which allows Ceph perform well in a degraded state.

osd recovery delay start
Description
After peering completes, Ceph will delay for the specified number of seconds before starting to recover objects.
Type
Float
Default
0
osd recovery max active
Description
The number of active recovery requests per OSD at one time. More requests will accelerate recovery, but the requests places an increased load on the cluster.
Type
32-bit Integer
Default
15
osd recovery max chunk
Description
The maximum size of a recovered chunk of data to push.
Type
64-bit Integer Unsigned
Default
8 << 20
osd recovery threads
Description
The number of threads for recovering data.
Type
32-bit Integer
Default
1
osd recovery thread timeout
Description
The maximum time in seconds before timing out a recovery thread.
Type
32-bit Integer
Default
30
osd recover clone overlap
Description
Preserves clone overlap during recovery. Should always be set to true.
Type
Boolean
Default
true

6.8. Miscellaneous

osd snap trim thread timeout
Description
The maximum time in seconds before timing out a snap trim thread.
Type
32-bit Integer
Default
60*60*1
osd backlog thread timeout
Description
The maximum time in seconds before timing out a backlog thread.
Type
32-bit Integer
Default
60*60*1
osd default notify timeout
Description
The OSD default notification timeout (in seconds).
Type
32-bit Integer Unsigned
Default
30
osd check for log corruption
Description
Check log files for corruption. Can be computationally expensive.
Type
Boolean
Default
false
osd remove thread timeout
Description
The maximum time in seconds before timing out a remove OSD thread.
Type
32-bit Integer
Default
60*60
osd command thread timeout
Description
The maximum time in seconds before timing out a command thread.
Type
32-bit Integer
Default
10*60
osd command max records
Description
Limits the number of lost objects to return.
Type
32-bit Integer
Default
256
osd auto upgrade tmap
Description
Uses tmap for omap on old objects.
Type
Boolean
Default
true
osd tmapput sets users tmap
Description
Uses tmap for debugging only.
Type
Boolean
Default
false
osd preserve trimmed log
Description
Preserves trimmed log files, but uses more disk space.
Type
Boolean
Default
false