Notice

This document is for a development version of Ceph.

Stretch Clusters

A stretch cluster spreads hosts across geographically separated data centers, with specific internal strategies to robustly localize network traffic and respond to failures. Stretch clusters have a higher likelihood of (possibly asymmetric) network splits, and a higher likelihood of temporary or complete loss of an entire data center (which can represent one-third to one-half of the total cluster).

Ceph is designed with the expectation that all parts of its network and cluster will be reliable and that failures will be distributed randomly across the CRUSH topology. When a host or network switch goes down, many OSDs will become unavailable. Ceph is designed so that the remaining OSDs and Monitors will maintain access to data.

Sometimes this cannot be relied upon. If you have a “stretched-cluster” deployment in which much of your cluster is behind a single network component, you might need to use the explicit stretch mode to ensure data integrity.

We will here consider two standard configurations: a configuration with two data centers (or, in clouds, two availability zones), and a configuration with three data centers.

In the two-site configuration, Ceph arranges for each site to hold a copy of the data. A third site houses a tiebreaker (arbiter, witness) Monitor. This tiebreaker Monitor picks a winner when a network connection between sites fails and both data centers remain alive.

The tiebreaker monitor can be a VM. It can also have higher network latency to the OSD site(s) than OSD site(s) can have to each other.

The standard Ceph configuration is able to survive many network failures or data-center failures without compromising data availability. When enough cluster components are brought back following a failure, the cluster will recover. If you lose a data center but are still able to form a quorum of monitors and still have replicas of all data available, Ceph will maintain availability. This assumes that the cluster has enough copies to satisfy the pools’ min_size configuration option, or (failing that) that the cluster has CRUSH rules in place that will cause the cluster to re-replicate the data until the min_size configuration option has been met.

Stretch Cluster Issues

Ceph does not compromise data integrity and data consistency under any circumstances. When service is restored after a network failure or a loss of Ceph nodes, Ceph will return to a state of normal function without human intervention.

Ceph does not permit the compromise of data integrity or data consistency, but there are situations in which data availability is compromised. These situations can occur even though there are sufficient replicas of data available to satisfy consistency and sizing constraints. In some situations, you might discover that your cluster does not satisfy those constraints.

The first category of these failures that we will discuss involves inconsistent networks. If there is a netsplit (a failure that splits the network into two conceptual islands that cannot communicate with each other), Ceph might be unable to mark OSDs down and remove them from Placement Group (PG) acting sets. This failure to mark ODSs down will occur despite the fact that the primary PG is unable to replicate data (a situation that, under normal non-netsplit circumstances, would result in the marking of affected OSDs as down and their removal from the PG). When this happens, Ceph will be unable to satisfy durability guarantees and consequently IO will not be permitted.

The second category of failures that we will discuss are those in which the constraints are not sufficient to guarantee the replication of data across data centers, though it might seem that the data is correctly replicated. For example, in a scenario in which there are two data centers named Data Center A and Data Center B, and a CRUSH rule targets three replicas and places a replica in each data center for a pool with min_size=2, the PG might go active with two replicas in Data Center A and zero replicas in Data Center B. In this situation, the loss of Data Center A means that the data is unavailable and Ceph and clients will not be able to operate on it. This situation is difficult to avoid using only conventional CRUSH rules.

Individual Stretch Pools

Setting individual stretch pool attributes allows for specific pools to be distributed across two or more data centers. This is done by executing the ceph osd pool stretch set command on each desired pool. See Unsetting values for a stretch pool

Use stretch mode when you have exactly two data centers and require a uniform configuration across the entire cluster. Conversely, opt for a stretch pool when you need only a particular pool to be replicated across more than two data centers, providing a more granular level of control.

Limitations

Individual stretch pools do not support I/O operations during a netsplit scenario between two or more zones. While the cluster remains accessible for basic Ceph commands, I/O remains unavailable until the netsplit is resolved. This is different from stretch mode, where the tiebreaker monitor can isolate one CRUSH datacenter and serve I/O operations in degraded mode during a netsplit. See Stretch Mode

Ceph is designed to tolerate multiple component failures. However, if more than 25% of the OSDs in the cluster go down, Ceph may stop marking OSDs out, which prevents rebalancing and may result in PGs becoming inactive. This behavior is controlled by the mon_osd_min_in_ratio option. The default value is 0.75, meaning that at least 75% of the OSDs in the cluster must be active for any additional OSDs to be marked out. This setting prevents too many OSDs from being marked out as this might lead to cascading failures and an impactful thundering herd of data movement. This can cause substantial client impact and long recovery times when OSDs return to service. If Ceph stops marking OSDs out, some PGs may fail to rebalance to surviving OSDs, potentially leading to inactive PGs. See https://tracker.ceph.com/issues/68338 for more information.

Stretch Mode

Stretch mode is designed to handle netsplit scenarios between two data centers as well as the loss of one data center. It handles the netsplit scenario by choosing the surviving zone that has the best connection to the tiebreaker Monitor. It handles the loss of one data center by reducing the min_size of all pools to 1, allowing the cluster to continue operating within the surviving data center. When the unavailable data center comes back, Ceph will converge according to configured replication policy and return to normal operation.

Connectivity Monitor Election Strategy

When using stretch mode, the Monitor election strategy must be set to connectivity. This strategy tracks network connectivity between Monitors and is used to determine which data center should be favored when the cluster experiences netsplit.

See Changing Monitor Elections

Stretch Peering Rule

One critical behavior of stretch mode is its ability to prevent a PG from going active if the acting set contains only replicas from a single data center. This safeguard is crucial for mitigating the risk of data loss during site failures because if a PG were allowed to go active with replicas only at a single site, writes could be acknowledged despite a lack of redundancy. In the event of a site failure, all data in the affected PG would be lost.

Entering Stretch Mode

To enable stretch mode, you must set the location of each monitor, correlating with the CRUSH topology.

Place mon.a in your first data center:

ceph mon set_location a datacenter=site1

Generate a CRUSH rule that places two copies in each data center. This requires editing the CRUSH map directly:
```
ceph osd getcrushmap > crush.map.bin
crushtool -d crush.map.bin -o crush.map.txt
```
Edit the crush.map.txt file to add a new rule. Here there is only one other rule (id 1), but you will likely need to use a different, unique rule ID. We have two datacenter buckets named site1 and site2:
```
 rule stretch_rule {
        id 1
        type replicated
        step take site1
        step chooseleaf firstn 2 type host
        step emit
        step take site2
        step chooseleaf firstn 2 type host
        step emit
}
```
Warning

When a CRUSH rule is defined in a stretch mode cluster and the rule has multiple take steps, MAX AVAIL for the pools associated with the CRUSH rule will report that the available size is all of the available space from the datacenter, not the available space for the pools associated with the CRUSH rule.

For example, consider a cluster with two CRUSH rules, stretch_rule and stretch_replicated_rule:
```
rule stretch_rule {
     id 1
     type replicated
     step take DC1
     step chooseleaf firstn 2 type host
     step emit
     step take DC2
     step chooseleaf firstn 2 type host
     step emit
}

rule stretch_replicated_rule {
        id 2
        type replicated
        step take default
        step choose firstn 0 type datacenter
        step chooseleaf firstn 2 type host
        step emit
}
```
In the above example, stretch_rule will report an incorrect value for MAX AVAIL. stretch_replicated_rule will report the correct value. This is because stretch_rule is defined in such a way that PGMap::get_rule_avail considers only the available capacity of a single datacenter, and not (as would be correct) the total available capacity from both datacenters.

Here is a workaround. Instead of defining the stretch rule as defined in the stretch_rule above, define it as follows:
```
rule stretch_rule {
  id 2
  type replicated
  step take default
  step choose firstn 0 type datacenter
  step chooseleaf firstn 2 type host
  step emit
}
```
See https://tracker.ceph.com/issues/56650 for more detail on this workaround.
The above procedure was developed in May and June of 2024 by Prashant Dhange.

Compile and inject the CRUSH map to make the rule available to the cluster:

crushtool -c crush.map.txt -o crush2.map.bin
ceph osd setcrushmap -i crush2.map.bin

Run the Monitors in connectivity mode. See Changing Monitor Elections.
```
ceph mon set election_strategy connectivity
```
Direct the cluster to enter stretch mode. In this example, mon.e is the tiebreaker Monitor and we are splitting across CRUSH datacenters. The tiebreaker monitor must be assigned a CRUSH datacenter that is neither site1 nor site2. This data center should not be predefined in your CRUSH map. Here we are placing mon.e in a virtual data center named site3:
```
ceph mon set_location e datacenter=site3
ceph mon enable_stretch_mode e stretch_rule datacenter
```

When stretch mode is enabled, PGs will become active only when they peer across CRUSH datacenter``s (or across whichever CRUSH bucket type was specified), assuming both are available. Pools will increase in size from the default ``3 to 4, and two replicas will be placed at each site. OSDs will be allowed to connect to Monitors only if they are in the same data center as the Monitors. New Monitors will not be allowed to join the cluster if they do not specify a CRUSH location.

If all OSDs and Monitors in one of the datacenter``s become inaccessible at once, the cluster in the surviving ``datacenter enters degraded stretch mode. A health state warning will be raised, pools’ min_size will be reduced to 1, and the cluster will be allowed to go active with the components and data at the single remaining site. Pool size does not change, so warnings will be raised that the PGs are undersized, but a special stretch mode flag will prevent the OSDs from creating extra copies in the remaining data center. This means that the data center will keep only two copies, just as before.

When the inaccessible datacenter comes back, the cluster will enter recovery stretch mode. This changes the warning and allows peering, but requires OSDs only from the datacenter that was up throughout the duration of the downtime. When all PGs are in a known state, and are neither degraded nor undersized / incomplete, the cluster transitions back to regular stretch mode, ends the warning, restores pools’ min_size to its original value of 2, requires PGs at both sites to peer, and no longer requires the site that was up throughout the duration of the downtime when peering. This makes failover to the other site possible, if needed.

Exiting Stretch Mode

To exit stretch mode, run the following command:

ceph mon disable_stretch_mode [{crush_rule}] --yes-i-really-mean-it

{crush_rule}

The non-stretch CRUSH rule to use for all pools. If this is not specified, the pools will move to the default CRUSH rule.

Type:: String
Required:: No.

This command moves the cluster back to normal mode; the cluster will no longer be in stretch mode. All pools will be set with their prior size and min_size values. At this point the user is responsible for scaling down the cluster to the desired number of OSDs if they choose to operate with fewer OSDs.

Note that the command will not execute when the cluster is in recovery stretch mode. The command executes only when the cluster is in degraded stretch mode or healthy stretch mode.

Limitations of Stretch Mode

When using stretch mode, OSDs must be located at exactly two sites.

Two Monitors must be run in each data center, plus a tiebreaker in a third (possibly in the cloud) for a total of five Monitors. While in stretch mode, OSDs will connect only to Monitors within the data center in which they are located. OSDs DO NOT connect to the tiebreaker monitor.

Erasure-coded pools cannot be used with stretch mode. Attempts to use erasure coded pools with stretch mode will fail. Erasure coded pools cannot be created while in stretch mode.

To use stretch mode, you will need to create a CRUSH rule that provides two replicas in each data center. Ensure that there are four total replicas: two in each data center. If pools exist in the cluster that do not have the default size or min_size, Ceph will not enter stretch mode. An example of such a CRUSH rule is given above.

Because stretch mode runs with poos’ min_size set to 1 , we recommend enabling stretch mode only when using OSDs on SSDs. Hybrid HDD+SSD or HDD-only OSDs are not recommended due to the long time it takes for them to recover after connectivity between data centers has been restored. This reduces the potential for data loss.

Warning

CRUSH rules that specify a device class are not supported in stretch mode. For example, the following rule specifying the ssd device class will not work:

rule stretch_replicated_rule {
           id 2
           type replicated class ssd
           step take default
           step choose firstn 0 type datacenter
           step chooseleaf firstn 2 type host
           step emit
}

In the future, stretch mode could support erasure-coded pools, enable deployments across more than two data centers, and accommodate multiple CRUSH device classes.

Other commands

Replacing a failed tiebreaker monitor

Deploy a new Monitor and run the following command:

ceph mon set_new_tiebreaker mon.<new_mon_name>

This command protests if the new Monitor is in the same CRUSH location as the existing, non-tiebreaker monitors. This command will not remove the previous tiebreaker monitor. If appropriate, you must remove the previous tiebreaker Monitor manually.

Using “--set-crush-location” and not “ceph mon set_location”

If you employ your own tooling for deploying Ceph, use the --set-crush-location option when booting Monitors instead of running ceph mon set_location. This option accepts only a single bucket=loc parameter, for example ceph-mon --set-crush-location 'datacenter=a', and that parameter’s CRUSH bucket type must match the bucket type that was specified when running enable_stretch_mode.

Forcing recovery stretch mode

When in stretch degraded mode, the cluster will go into recovery mode automatically when the disconnected data center comes back. If that does not happen or you want to enable recovery mode early, run the following command:

ceph osd force_recovery_stretch_mode --yes-i-really-mean-it

Forcing normal stretch mode

When in recovery mode, the cluster should go back into normal stretch mode when the PGs are healthy. If this fails to happen or if you want to force cross-data-center peering early and are willing to risk data downtime (or have verified separately that all the PGs can peer, even if they aren’t fully recovered), run the following command:

ceph osd force_healthy_stretch_mode --yes-i-really-mean-it

This command can be used to to remove the HEALTH_WARN state, which recovery mode raises.

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