nbdkit-retry-filter(1)

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
Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

•   Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

•   Redistributions in binary form must reproduce the above copyright notice, this list of conditions and 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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