Comment 13 for bug 1666320

Reviewed: https://review.opendev.org/738959
Committed: https://git.openstack.org/cgit/openstack/liberasurecode/commit/?id=5626cd5791bd35279594e067c10581e89ae66abc
Submitter: Zuul
Branch: master

commit 5626cd5791bd35279594e067c10581e89ae66abc
Author: Tim Burke <email address hidden>
Date: Wed Jul 1 21:59:45 2020 -0700

    Be willing to write fragments with legacy crc

    ...if users *really* want to. They opt-in at run time by setting

        LIBERASURECODE_WRITE_LEGACY_CRC=1

    in the environment; leaving it unset, set to an empty string, or set to
    the string "0" continues to write zlib crcs.

    UpgradeImpact
    =============
    This option is intended to allow a smooth upgrade from liberasurecode
    1.5.0 and earlier in a system with multiple readers and writers:

      * Before upgrade, ensure the environment variable is set on all nodes.
        This will be ignored by earlier versions.
      * Upgrade liberasurecode on each node in the system, restarting any
        services that use it. Every node continues writing CRCs that are
        still usable by nodes that have not yet upgraded.
      * Now that every node is capable of reading zlib CRCs, remove the
        environment variable from each node to start writing new CRCs.

    If you are already using 1.6.0 or later, just upgrade normally.

    Closes-Bug: #1886088
    Closes-Bug: #1867937
    Related-Bug: #1666320
    Needed-By: https://review.opendev.org/#/c/739164/
    Change-Id: I9adfbe631a2dddc592fd08f8a325f3e8331b92f1