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When a file is transfered into dCache its replica is copied into one of the pools. Since this is the only replica and normally the required range is higher (e.g., by default at least 2 and at most 3), this file will be replicated to other pools.

When some pools go down, the replica count for the files in these pools may fall below the valid range and these files will be replicated. Replicas of the file with replica count below the valid range and which need replication are called deficient replicas.

Later on some of the failed pools can come up and bring online more valid replicas. If there are too many replicas for some file these extra replicas are called redundant replicas and they will be reduced. Extra replicas will be deleted from pools.

The replica service counts the number of replicas for each file in the pools which can be used online (see Pool States below) and keeps the number of replicas within the valid range (replica.limits.replicas.min, replica.limits.replicas.max).

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Pool States

The possible states of a pool are online, down, offline, offline-prepare and drainoff. They can be set by the admin through the admin interface. (See the section called “Commands for the admin interface”.)

Figure 6.1. Pool State Diagram

Pool State Diagram


Normal operation.

Replicas in this state are readable and can be counted. Files can be written (copied) to this pool.


A pool can be down because

  • the admin stopped the domain in which the pool was running.
  • the admin set the state value via the admin interface.
  • the pool crashed

To confirm that it is safe to turn pool down there is the command ls unique in the admin interface to check number of files which can be locked in this pool. (See the section called “Commands for the admin interface”.)

Replicas in pools which are down are not counted, so when a pool crashes the number of online replicas for some files is reduced. The crash of a pool (pool departure) may trigger replication of multiple files.

On startup, the pool comes briefly to the online state, and then it goes down to do pool Inventory to cleanup files which broke when the pool crashed during transfer. When the pool comes online again, the replica service will update the list of replicas in the pool and store it in the database.

Pool recovery (arrival) may trigger massive deletion of file replicas, not necessarily in this pool.


The admin can set the pool state to be offline. This state was introduced to avoid unnecessary massive replication if the operator wants to bring the pool down briefly without triggering massive replication.

Replicas in this pool are counted, therefore it does not matter for replication purpose if an offline pool goes down or up.

When a pool comes online from an offline state replicas in the pool will be inventoried to make sure we know the real list of replicas in the pool.


This is a transient state betweeen online and offline.

The admin will set the pool state to be offline-prepare if he wants to change the pool state and does not want to trigger massive replication.

Unique files will be evacuated — at least one replica for each unique file will be copied out. It is unlikely that a file will be locked out when a single pool goes down as normally a few replicas are online. But when several pools go down or set drainoff or offline file lockout might happen.

Now the admin can set the pool state offline and then down and no file replication will be triggered.


This is a transient state betweeen online and down.

The admin will set the pool state to be drainoff if he needs to set a pool or a set of pools permanently out of operation and wants to make sure that there are no replicas locked out.

Unique files will be evacuated — at least one replica for each unique file will be copied out. It is unlikely that a file will be locked out when a single pool goes down as normally a few replicas are online. But when several pools go down or set drainoff or offline file lockout might happen.

Now the admin can set the pool state down. Files from other pools might be replicated now, depending on the values of replica.limits.replicas.min and replica.limits.replicas.max.

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When the replica service starts it cleans up the database. Then it waits for some time to give a chance to most of the pools in the system to connect. Otherwise unnecessary massive replication would start. Currently this is implemented by some delay to start adjustments to give the pools a chance to connect.

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Cold Start

Normally (during Cold Start) all information in the database is cleaned up and recreated again by polling pools which are online shortly after some minimum delay after the replica service starts. The replica service starts to track the pools’ state (pool up/down messages and polling list of online pools) and updates the list of replicas in the pools which came online. This process lasts for about 10-15 minutes to make sure all pools came up online and/or got connected. Pools which once get connected to the replica service are in online or down state.

It can be annoying to wait for some large period of time until all known good pools get connected. There is a Hot Restart option to accelerate the restart of the system after the crash of the head node.

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Hot Restart

On Hot Restart the replica service retrieves information about the pools’ states before the crash from the database and saves the pools’ states to some internal structure. When a pool gets connected the replica service checks the old pool state and registers the old pool’s state in the database again if the state was offline, offline-prepare or drainoff state. The replica service also checks if the pool was online before the crash. When all pools which were online get connected once, the replica service supposes it recovered its old configuration and the replica service starts adjustments. If some pools went down during the connection process they were already accounted and adjustment would take care of it.


Suppose we have ten pools in the system, where eight pools were online and two were offline before a crash. The replica service does not care about the two offline pools to get connected to start adjustments. For the other eight pools which were online, suppose one pool gets connected and then it goes down while the other pools try to connect. The replica service considers this pool in known state, and when the other seven pools get connected it can start adjustments and does not wait any more.

If the system was in equilibrium state before the crash, the replica service may find some deficient replicas because of the crashed pool and start replication right away.

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Avoid replicas on the same host

For security reasons you might want to spread your replicas such that they are not on the same host, or in the same building or even in the same town. To configure this you need to set the tag.hostname label for your pools and check the properties replica.enable.check-pool-host and replica.enable.same-host-replica.


We assume that some pools of your dCache are in Hamburg and some are in Berlin. In the layout files where the respective pools are defined you can set




By default the property replica.enable.check-pool-host is true and replica.enable.same-host-replica is false. This means that the tag.hostname will be checked and the replication to a pool with the same tag.hostname is not allowed.

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Hybrid dCache

A hybrid dCache operates on a combination of pools (maybe connected to tape) which are not in a resilient pool group and the set of resilient pools. The replica service takes care only of the subset of pools configured in the pool group for resilient pools and ignores all other pools.


If a file in a resilient pool is marked precious and the pool were connected to a tape system, then it would be flushed to tape. Therefore, the pools in the resilient pool group are not allowed to be connected to tape.

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Commands for the admin interface

If you are an advanced user, have proper privileges and you know how to issue a command to the admin interface you may connect to the ReplicaManager cell and issue the following commands. You may find more commands in online help which are for debug only — do not use them as they can stop replica service operating properly.

set pool <pool><state>

set pool state

show pool <pool>

show pool state

ls unique <pool>

Reports number of unique replicas in this pool.

exclude <pnfsId>

exclude <pnfsId> from adjustments

release <pnfsId>

removes transaction/BAD status for <pnfsId>

debug true | false

enable/disable DEBUG messages in the log file