This is documentation for MapR Version 5.0. You can also refer to MapR documentation for the latest release.

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When a disk fails, MapR raises the node-level alarm NODE_ALARM_DISK_FAILURE on the node with the failed disk (or disks). At the same time, other disks in the same storage pool as the failed disk are taken offline. You can look at the MapR Control System (MCS) and click on Cluster>Dashboard to see a cluster heatmap of each node and a list of alarms, similar to this:

By hovering your mouse over the , you can get more information about the reason for the failure. By clicking on the , you can display node-specific information including an alarm summary like the one below:

When you see a disk failure alarm, examine the log file at /opt/mapr/logs/faileddisk.log and check the Failure Reason field.

Examining the Cause of Failure

In the faileddisk.log file, you will see information on the cause of failure. In the sample log output below, the failure reason is I/O error. Notice that the log file also provides instructions for removing disks and adding them back to MapR-FS.

Recovering from Failures

Most software failures can be remedied by running the fsck utility, which scans the storage pool that the disk belongs to and reports errors. For hardware failures, remove the failed disk and replace it according to the procedure in Removing and Replacing Disks.

The following table lists types of failures and recommended courses of action:

Failure ReasonRecommended Course of Action
I/O time outIncrease the value of the parameter in the /opt/mapr/conf/mfs.conf file.
No such deviceCheck if the disk has been renamed. If so, re-add this disk by running the disk add command.
I/O errorTest the drive for possible causes.
CRC errorRun fsck -n <sp> -d to perform a CRC (Cyclic Redundancy Check) on the data blocks in the storage pool, then bring it back online.
Slow diskTest the drive for possible causes.
GUID of disk mismatches with the one in $INSTALL_DIR/conf/disktab.
It's possible that disk names have changed.
After a node restart, the operating system can reassign the drive labels (for example, /sda), resulting in drive labels no longer matching the entries in the disktab file. Edit the disktab file according to the instructions in the log to repair the problem.
Unknown errorContact MapR support.

Addressing Data Alarms

When a disk fails, data on that disk becomes unavailable. As a result, you will probably see one of these two data alarms along with a Disk Failure alarm:

  • Data Unavailable (VOLUME_ALARM_DATA_UNAVAILABLE) - if there was only one copy of data and it was on the failed disk; or if data was replicated more than once, but all disks with that data failed
  • Data Under Replicated (VOLUME_ALARM_DATA_UNDER_REPLICATED) - if data on the failed disk is replicated elsewhere, but the minimum replication factor is not met as a result of the failed disk

If you see a Data Unavailable volume alarm in the cluster, follow these steps to run the /opt/mapr/server/fsck utility on all the offline storage pools. On each node in the cluster that has raised a disk failure alarm:

    1. Run the following command to identify which storage pools are offline:

      [user@host] /opt/mapr/server/mrconfig sp list | grep Offline
    2. For each storage pool reported by the previous command, run the following command, where <sp> specifies the name of an offline storage pool:

      [user@host] /opt/mapr/server/fsck -n <sp> -r 

      When you run fsck with the -r option, the fsck utility identifies corrupt blocks and removes them. If there are no corrupt blocks, fsck clears the error condition so you can bring the storage pool back online.


      Using the /opt/mapr/server/fsck utility with the -r flag to repair a filesystem risks data loss. Call MapR support before using /opt/mapr/server/fsck -r.

    3. Verify that all Data Unavailable volume alarms are cleared. If Data Unavailable volume alarms persist, contact MapR support or post on

If there are any Data Under Replicated volume alarms in the cluster, MapR can repair the problem by automatically replicating data and putting it on another disk. After you allow a reasonable amount of time for re-replication, verify that the under-replication alarms are cleared. 


Using the /opt/mapr/server/fsck utility with the -r option produces different results depending on the scenario. The fsck utility does not interpret the scenario nor does it have a safe mode.

  • If a disk is offline because of an imbalanced b-tree, using fsck -r may result in data loss from bad containers and data loss if additional replicas are unavailable.
  • If a disk is offline because of an I/O error, using fsck -r produces indeterminate results. A disk that is throwing I/O errors is questionable in terms of data content and reliability. For example, an operation that completed on the disk but was never returned may have partial data remaining on the disk. Using fsck -r retains any partial data.
  • If a disk is offline because of a slow I/O, using fsck -r does not produce data loss.

The most conservative usage of fsck -r is to run fsck without the -r option (verification mode) and check the output. If the output is ok, then run fsck with the -r option.



Disk Failure node alarms that persist require disk replacement. If Data Under Replicated volume alarms persist, contact MapR support or post on

Removing and Replacing Disks

If a disk fails due to a hardware problem, you will need to remove the disk. You can replace it, and then add that disk back to MapR-FS along with the other disks that were automatically removed at the same time.

To remove and replace disks using the MapR command-line interface:

  1. On the node with failed disks, determine which disk to replace by examining Disk entries in the /opt/mapr/logs/faileddisk.log file.
    In the sample log file above, the failed disk is /dev/sdd
  2. Use the disk remove command to remove the disk. Run the following command, substituting the hostname or IP address for <host> and a list of failed disks for <disks>:

    The disk removal process can take several minutes to complete. 

  3. Examine the screen output in response to this command:

    Note the additional disks removed when /dev/sdd is removed. The disks /dev/sde and /dev/sdf are part of the same storage pool and therefore removed along with the failed disk.
  4. Confirm that removed disks do not appear in the disktab file.
  5. Replace the failed disks on the node or nodes, following correct procedures for your hardware.
  6. Remove the failed disk log file from the /opt/mapr/logs directory. These log files are typically named in the pattern

  7. Use the disk add command to add the replacement disk (or disks) along with other disks from the same storage pool or pools.


    This step reformats the disks. Any data on these disks will be lost.

    For example, to add the removed disks shown in step 3, the command is:

    Note that when the failed disk has been replaced, it should be added back to MapR-FS along with the other disks from the same storage pool that were previously removed. If you add only the replacement disk to MapR-FS, this will result in a non-optimal storage pool layout which can lead to degraded performance.

    Once the disks are added to MapR-FS, the cluster allocates properly-sized storage pools automatically. For example, if you add ten disks, MapR allocates two storage pools of three disks each and two storage pools of two disks each.

To remove and replace disks using the MapR Control System:

  1. Identify the failed disk or disks:
    • In the Navigation pane, expand the Cluster group and click the Nodes view.
    • Click the name of the node on which you want to replace disks, and look in the MapR-FS and Available Disks pane.
  2. Remove the failed disk or disks from MapR-FS:
    • In the MapR-FS and Available Disks pane, click the checkbox that corresponds to each failed disk (with status ).
    • Click the Remove Disk(s) to MapR-FS button.
    • In the dialog box that opens, click OK.
    • Wait several minutes while the removal process completes. After you remove the disks, the offline disks from the same storage pools are marked as available (not in use by MapR).
    • From a command line terminal, remove the failed disk log file from the /opt/mapr/logs directory. These log files are typically named like this:
  3. Replace the failed disks on the node or nodes according to the correct hardware procedure.
  4. Add the replacement and available disks to MapR-FS:
    • In the Navigation pane, expand the Cluster group and click the Nodes view.
    • Click the name of the node on which you replaced the disks.
    • In the MapR-FS and Available Disks pane, click the checkboxes beside the disks you want to add to the storage pool.
    • Click the Add Disk(s) to MapR-FS button.
    • When the confirmation dialog box appears, click OK.
  5. Note that the display shows MapR-FS under the File System column, which indicates that the disks were successfully added.
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