plucky (1) nbdkit-readahead-filter.1.gz

Provided by: nbdkit_1.42.0-1ubuntu1_amd64 bug

NAME
       nbdkit-readahead-filter - prefetch data ahead of sequential reads


SYNOPSIS
        nbdkit --filter=readahead PLUGIN

        nbdkit --filter=readahead --filter=cache PLUGIN

        nbdkit --filter=readahead --filter=cow PLUGIN cow-on-cache=true


DESCRIPTION
       "nbdkit-readahead-filter" is a filter that prefetches data when the client is reading.

       When the client issues a read, this filter issues a parallel prefetch (".cache") for subsequent data.
       Plugins which support this command will prefetch the data, making sequential reads faster.  For plugins
       which do not support this command, you can inject nbdkit-cache-filter(1) below (after) this filter,
       giving approximately the same effect.  nbdkit-cow-filter(1) can be used instead of nbdkit-cache-filter,
       if you add the "cow-on-cache=true" option.

       The filter uses a simple adaptive algorithm which accelerates sequential reads and requires no further
       configuration.

       A similar filter is nbdkit-scan-filter(1) which reads ahead over the whole disk, useful if you know that
       the client will be reading sequentially across most or all of the disk.

   Limitations
       In a number of significant cases this filter will do nothing.  The filter will print a warning message if
       this happens.

       Thread model must be parallel
           Use "nbdkit PLUGIN --dump-plugin" to find the thread model of the underlying plugin.  If it does not
           support the "parallel" thread model, this filter cannot perform prefetches in parallel with the read
           requests.

           We may be able to lift this restriction in future.

       Underlying filters or plugin must support ".cache" (prefetch)
           Very many plugins do not have the concept of prefetching and/or do not implement the ".cache"
           callback, and so there is no way for this filter to issue prefetches.

           You can usually get around this by adding --filter=cache after this filter as explained above.  It
           may be necessary to limit the total size of the cache (see "CACHE MAXIMUM SIZE" in
           nbdkit-cache-filter(1)).

       Clients and kernels may do readahead already
           It may be the case that NBD clients are already issuing "NBD_CMD_CACHE" (NBD prefetch) commands.  It
           may also be the case that your plugin is using local file functions where the kernel is doing
           readahead.  In such cases this filter is not necessary and may be pessimal.


PARAMETERS
       There are no parameters specific to nbdkit-readahead-filter.  Any parameters are passed through to and
       processed by the underlying plugin in the normal way.


FILES
       $filterdir/nbdkit-readahead-filter.so
           The filter.

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


VERSION
       "nbdkit-readahead-filter" first appeared in nbdkit 1.12.


SEE ALSO
       nbdkit(1), nbdkit-cache-filter(1), nbdkit-cow-filter(1), nbdkit-file-plugin(1), nbdkit-retry-filter(1),
       nbdkit-scan-filter(1), nbdkit-torrent-plugin(1), nbdkit-vddk-plugin(1), nbdkit-filter(3), qemu-img(1).


AUTHORS
       Richard W.M. Jones


       Copyright Red Hat


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.

       THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
       INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
       PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
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