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.

       An alternative and more fine-grained filter is nbdkit-retry-request-filter(1) which will
       retry only single requests that fail.

       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), nbdkit-retry-request-filter(1).

AUTHORS

       Richard W.M. Jones

COPYRIGHT

       Copyright (C) 2019 Red Hat Inc.

LICENSE

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