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

       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,
       INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,  PROCUREMENT  OF
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       ON  ANY  THEORY  OF  LIABILITY,  WHETHER  IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
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       DAMAGE.

nbdkit-1.42.2                                      2025-04-02                         nbdkit-readahead-filter(1)