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 (C) 2019-2022 Red Hat Inc.

LICENSE

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