Always Disable “Over Commit” Before Creating LUNs When Using StorNext with Q-Tier

Why Disable "Over Commit" When Using StorNext and Q-Tier?

When you use StorNext with Quantum’s QXS Q-Tier, always disable the “over commit” feature before you create any LUNs. If “over commit” is enabled, it is possible to overprovision the LUNs and unknowingly run out of disk space. This could result in the loss of files that were written into the LBA range that was not provisioned with physical storage. This article provides some guidelines and recommendations.

Notes on Q-Tier Capacity Allocation and Utilization

Q-Tier determines used capacity based on used pages in the virtual LUN, and it determines which pages are used based on which pages are allocated. Pages are allocated when being written to by StorNext.

Q-Tier determines which pages are freed based on a file system sending the array the SCSI “UNMAP” command for areas on the disk with deleted files. Pages are de-allocated when the file system sends the UNMAP command for those LBA ranges contained on a page. StorNext does not support the SCSI UNMAP command and therefore cannot tell the QXS which previously allocated areas of the disk have been freed.

The implications of no support for SCSI UNMAP in StorNext are as follows: 

  1. StorNext will always have accurate free and used space calculations.
  2. Assuming that the “over-commit” feature is disabled, lack of support for SCSI UNMAP is harmless, except that QXS will have inaccurate free and used space calculations.
  3. StorNext will efficiently reuse the space allocated to it.
  4. The QXS array will inaccurately report the used pages in the QXS GUI. As shown below, by disabling “over commit” you can avoid negative effects on performance that this can cause.

Notes on Q-Tier Capacity Monitoring and Alerts

For the reasons stated above, using QXS with Q-Tier will produce inaccurate used/free capacity calculations.

There are three capacity threshold alerts associated with each pool (low, medium, high). When the available capacity goes below one of the thresholds, an alert is sent out.

Because the QXS array’s capacity calculations may be inaccurate when it is used with StorNext, “over commit” must always be disabled when virtual LUNs are used with StorNext, to avoid diminished performance:

  • If “over commit” is enabled, when the number of available pages goes below the lowest threshold, the array automatically switches to "write-through" mode to protect the integrity of each write operation. This will diminish the write performance of the array.
  • If “over commit” is disabled, the array will not go into “write-through” mode when the capacity threshold is reached, so performance will not be diminished.
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