This document is for a development version of Ceph.

Stretch Clusters

Stretch Clusters

A stretch cluster is a cluster that has servers in geographically separated data centers, distributed over a WAN. Stretch clusters have LAN-like high-speed and low-latency connections, but limited links. 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 map. Even if a switch goes down and causes the loss of many OSDs, Ceph is designed so that the remaining OSDs and monitors will route around such a loss.

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 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 (or, in clouds, three availability zones).

In the two-site configuration, Ceph expects each of the sites to hold a copy of the data, and Ceph also expects there to be a third site that has a tiebreaker monitor. This tiebreaker monitor picks a winner if the network connection fails and both data centers remain alive.

The tiebreaker monitor can be a VM. It can also have high latency relative to the two main sites.

The standard Ceph configuration is able to survive MANY network failures or data-center failures without ever compromising data availability. If enough Ceph servers 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 all the 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 permit the compromise of data integrity and data consistency under any circumstances. When service is restored after a network failure or a loss of Ceph nodes, Ceph will restore itself to a state of normal functioning without operator 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 enough clusters available to satisfy Ceph’s 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 disconnection between two servers that splits the network into two pieces), Ceph might be unable to mark OSDs down and remove them from the acting PG 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). If this happens, Ceph will be unable to satisfy its durability guarantees and consequently IO will not be permitted.

The second category of failures that we will discuss involves the situation 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 across data centers. For example, in a scenario in which there are two data centers named Data Center A and Data Center B, and the CRUSH rule targets three replicas and places a replica in each data center with a min_size of 2, the PG might go active with two replicas in Data Center A and zero replicas in Data Center B. In a situation of this kind, the loss of Data Center A means that the data is lost and Ceph will not be able to operate on it. This situation is surprisingly difficult to avoid using only standard CRUSH rules.

Stretch Mode

Stretch mode is designed to handle deployments in which you cannot guarantee the replication of data across two data centers. This kind of situation can arise when the cluster’s CRUSH rule specifies that three copies are to be made, but then a copy is placed in each data center with a min_size of 2. Under such conditions, a placement group can become active with two copies in the first data center and no copies in the second data center.

Entering Stretch Mode

To enable stretch mode, you must set the location of each monitor, matching your CRUSH map. This procedure shows how to do this.

  1. Place mon.a in your first data center:

    ceph mon set_location a datacenter=site1
  2. Generate a CRUSH rule that places two copies in each data center. This requires editing the CRUSH map directly:

    ceph osd getcrushmap >
    crushtool -d -o
  3. Edit the file to add a new rule. Here there is only one other rule (id 1), but you might need to use a different rule ID. We have two data-center 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
  4. Inject the CRUSH map to make the rule available to the cluster:

    crushtool -c -o
    ceph osd setcrushmap -i
  5. Run the monitors in connectivity mode. See Changing Monitor Elections.

    ceph mon set election_strategy connectivity
  6. Command the cluster to enter stretch mode. In this example, mon.e is the tiebreaker monitor and we are splitting across data centers. The tiebreaker monitor must be assigned a data center that is neither site1 nor site2. This data center should not be defined in your CRUSH map, here we are placing mon.e in a virtual data center called 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 data centers (or across whichever CRUSH bucket type was specified), assuming both are alive. Pools will increase in size from the default 3 to 4, and two copies will be expected in 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 location.

If all OSDs and monitors in one of the data centers become inaccessible at once, the surviving data center enters a “degraded stretch mode”. A warning will be issued, the min_size will be reduced to 1, and the cluster will be allowed to go active with the data in the single remaining site. The pool size does not change, so warnings will be generated that report that the pools are too small -- 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 missing data center comes back, the cluster will enter a “recovery stretch mode”. This changes the warning and allows peering, but requires OSDs only from the data center that was up throughout the duration of the downtime. When all PGs are in a known state, and are neither degraded nor incomplete, the cluster transitions back to regular stretch mode, ends the warning, restores min_size to its original value (2), requires both sites to peer, and no longer requires the site that was up throughout the duration of the downtime when peering (which makes failover to the other site possible, if needed).

Limitations of Stretch Mode

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

Two monitors should be run in each data center, plus a tiebreaker in a third (or 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 min_size set to 1 (or, more directly, min_size 1), we recommend enabling stretch mode only when using OSDs on SSDs (including NVMe OSDs). Hybrid HDD+SDD 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.

In the future, stretch mode might support erasure-coded pools and might support deployments that have more than two data centers.

Other commands

Replacing a failed tiebreaker monitor

Turn on 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 location as the existing non-tiebreaker monitors. This command WILL NOT remove the previous tiebreaker monitor. Remove the previous tiebreaker monitor yourself.

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

If you write 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 pair (for example, ceph-mon --set-crush-location 'datacenter=a'), and that pair 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 the 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 generates.

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