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Data Rewrite Tools

Sara Levy
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There are several features available that affect the way data is written on the cluster, including DBox High Availability, encryption and Similarity-Based Data Reduction. Enabling DBox HA and/or encryption automatically triggers a rewrite process that restripes data across cluster drives and/or rewrites data blocks. In the case of the Similarity feature, the rewrite is optional since you can choose intead to write only future data with the feature.

In addition to the above, VAST Cluster provides two tools for directly triggering a rewrite of data on the cluster's storage media:

  • Rewrite data. This option performs a defragmentation of all data on the cluster, restriping all the data across the cluster's drives to achieve optimal RAID efficiency.

  • Efficient rewrite data. This option defragments smaller stripes and restripes them to bigger stripes for increased storage efficiency. It also rewrites all data blocks with similarity-based data reduction, provided similarity is enabled on the cluster.

One of these processes may be beneficial to run and may be recommended after expanding a cluster to include additional DBoxes, especially two or more DBoxes. Following an expansion, a large imbalance of data between old and new DBox drives may lead to non-optimal RAID layouts. A rewrite can sometimes be beneficial to balance the data across the DBox drives and promote efficient RAID striping and hence efficient use of your capacity.

Impact of Rewrite

The following are important points to note about the drive layout rewrite:

  • The majority of data is typically rewritten during this rewrite and therefore the impact on storage media endurance is approximately similar to that of deleting all data on the cluster and writing it.

  • The rewrite proceeds as a background task that cannot be paused or stopped. In case of severe performance degradation, it may be possible for VAST Support to throttle the process and reduce the performance impact.

  • The rewrite may take a while, and may impact performance for workloads.

  • If expansions are planned, they should be done prior to rewriting so that the rewrite will utilize as many DBoxes as possible and minimize RAID overhead.

  • Rewrite is automatically triggered if and when you enable DBox High Availability. Therefore, do not run a rewrite before or after you enable DBox HA on a cluster, since that will perform an unnecessary additional rewrite and cause unnecessary impact on performance as well as storage media endurance.

  • DBox expansion is not available while the rewrite is in progress.

  • It is not recommended to disable Similarity during an efficient rewrite. Disabling Similarity during rewrite causes the rewrite process to run indefinitely although it will stop applying similarity to the data.

How to Start a Rewrite

Caution

We recommend consulting your VAST System Engineer to assist you in deciding whether to run a rewrite and which rewrite to run.

To start a rewrite from the VAST Web UI:

  1. In the VAST Web UI, open the Cluster tab of the Settings page. You can reach this by searching at the top left or from navigation menu on the left of the page.

  2. In the General Cluster Setup and Actions section, click Rewrite Data or Efficient Rewrite Data.

    A confirmation prompt is displayed.

  3. Click Yes if you are sure you would like to proceed.

    The rewrite begins and a progress bar appears at the top right of the page, reporting the current phase of the rewrite as it progresses and the percentage progress.

    During progress, the Rewrite Data and Efficient Rewrite Data buttons are disabled, and this message appears below them:

    Rewrite is currently in progress

    You can monitor the phases of the rewrite from the Activities page.

    When the process is complete, the rewrite buttons are reenabled.

Starting a Rewrite from the VAST CLI

  1. Run the cluster modify command with one of the following options:

    • --enable-rewrite. This option performs a defragmentation of all data on the cluster, restriping all the data across the cluster's drives, to achieve optimal storage efficiency.

    • --enable-efficient-rewrite. This option defragments smaller stripes and restripes them to bigger stripes for increased storage efficiency. It also rewrites all data blocks with similarity-based data reduction, provided similarity is enabled on the cluster.

    For example:

    vcli: admin> cluster modify --enable-rewrite

  2. Enter 'y' to confirm that you want to proceed.

    You are then warned:

    Rewrite may impact workloads while it is in progress. Stopping rewrite requires support intervention. DBox expansion will not be available during rewrite. Are you sure you want to proceed?

  3. Enter 'y' to confirm again.

    The rewrite begins.

  4. You can now monitor the progress of the rewrite. Enter the command cluster show. The command output includes the following fields:

    • Rewrite-phase. During the rewrite, one of the main phases appears here. The order of the phases is:

      1. INTERNAL_PRE_REWRITE

      2. DATA_REWRITE_PRE

      3. DATA_REWRITE_SCRUB

      4. DATA_REWRITE

      5. LAYOUT_REWRITE_PRE

      6. LAYOUT_REWRITE

      7. FINALIZE

    • Rewrite-progress. This shows the percentage progress of the current phase of the rewrite.

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