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
           For  example  nbdkit-curl-plugin(1)  only  supports  "serialize_requests",  and so 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
       SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
       ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT  LIABILITY,  OR  TORT  (INCLUDING  NEGLIGENCE  OR
       OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
       DAMAGE.

nbdkit-1.36.3                                      2024-03-31                         nbdkit-readahead-filter(1)