NAME

       nbdkit-retry-filter - reopen connection on error

SYNOPSIS

        nbdkit --filter=retry PLUGIN [retries=N] [retry-delay=N]
                                     [retry-exponential=yes|no]
                                     [retry-readonly=yes|no]

DESCRIPTION

       "nbdkit-retry-filter" is a filter that transparently reopens the plugin connection when an error is
       encountered.  It can be used to make long-running copy operations reliable in the presence of temporary
       network failures, without requiring any changes to the plugin or the NBD client.

       Several optional parameters are available to control:

       •   how many times we retry,

       •   the delay between retries, and whether we wait longer each time (known as “exponential back-off”),

       •   if we reopen the plugin in read-only mode after the first failure.

       The default (with no parameters) is designed to offer a happy medium between recovering from short
       temporary failures but not doing anything too bad when permanent or unrecoverable failures happen.  The
       default behaviour is: we retry 5 times with exponential back-off, waiting in total about 1 minute before
       we give up.

EXAMPLE

       In this example we copy and convert a large file using nbdkit-ssh-plugin(1), qemu-img(1) and
       nbdkit-captive(1).

        nbdkit -U - \
          ssh host=remote.example.com /var/tmp/test.iso \
          --filter=retry \
          --run 'qemu-img convert -p -f raw $nbd -O qcow2 test.qcow2'

       Without --filter=retry a temporary failure would cause the copy to fail (for example, the remote host’s
       firewall is restarted causing the SSH connection to be dropped).  Adding this filter means that it may be
       possible to transparently recover.

PARAMETERS

       retries=N
           The number of times any single operation will be retried before we give up and fail the operation.
           The default is 5.

       retry-delay=N
           The number of seconds to wait before retrying.  The default is 2 seconds.

       retry-exponential=yes
           Use exponential back-off.  The retry delay is doubled between each retry.  This is the default.

       retry-exponential=no
           Do not use exponential back-off.  The retry delay is the same between each retry.

       retry-readonly=yes
           As soon as a failure occurs, switch the underlying plugin to read-only mode for the rest of this
           connection.  (A new NBD client connection will still open the plugin in the original mode.)

       retry-readonly=no
           Do not change the read-write/read-only mode of the plugin when retrying.  This is the default.

FILES

       $filterdir/nbdkit-retry-filter.so
           The filter.

           Use "nbdkit --dump-config" to find the location of $filterdir.

VERSION

       "nbdkit-retry-filter" first appeared in nbdkit 1.16.

SEE ALSO

       nbdkit(1), nbdkit-filter(3), nbdkit-readahead-filter(1).

AUTHORS

       Richard W.M. Jones

COPYRIGHT

       Copyright (C) 2019 Red Hat Inc.

LICENSE

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       that the following conditions are met:

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           the following disclaimer in the documentation and/or other materials provided with the distribution.

       •   Neither the name of Red Hat nor the names of its contributors may be used to endorse or promote
           products derived from this software without specific prior written permission.

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