NAME

       nbdkit-readahead-filter - prefetch data when reading sequentially

SYNOPSIS

        nbdkit --filter=readahead plugin

DESCRIPTION

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

       A common use for this filter is to accelerate sequential copy operations (like
       "qemu-img convert") when plugin requests have a high overhead (like
       nbdkit-curl-plugin(1)).  For example:

        nbdkit -U - --filter=readahead curl https://example.com/disk.img \
               --run 'qemu-img convert $nbd disk.img'

       The filter uses a simple adaptive algorithm which accelerates sequential reads, but has a
       small penalty if the client does random reads.  If the client mixes reads with writes or
       write-like operations (trimming, zeroing) then it will work but there can be a large
       performance penalty.

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-curl-plugin(1), nbdkit-retry-filter(1),
       nbdkit-ssh-plugin(1), nbdkit-vddk-plugin(1), nbdkit-filter(3), qemu-img(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:

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           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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