NAME

       nbdkit-scan-filter - scan disk prefetching data ahead of sequential reads

SYNOPSIS

        nbdkit --filter=scan PLUGIN [scan-ahead=false] [scan-clock=false]
                                    [scan-forever=true] [scan-size=]NN

        nbdkit --filter=scan --filter=cache PLUGIN

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

DESCRIPTION

       "nbdkit-scan-filter" is a filter that scans the disk prefetching data.  It is sometimes
       useful if you expect that the client will read the disk sequentially.

       The basic operation of the filter is that when a client connects, the filter will start
       issuing ".cache" (prefetch) requests to the plugin across the whole disk.  Plugins which
       support this command will prefetch the data, making subsequent 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.

       Various parameters can be used to tune scanning, although the defaults should be suitable
       in most cases.

       A similar filter is nbdkit-readahead-filter(1).

   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.

       Only scans while clients are connected*
           The current filter only scans while there is at least one client connected.

       Only scans the default export*
           The current filter only scans the default export and ignores all clients connecting to
           the non-default export name.

           *We may be able to lift these restrictions 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.

       Prefetching the whole disk may load it all into cache
           In particular if you use this filter together with nbdkit-cache-filter(1) or
           nbdkit-cow-filter(1), they will cache the whole content of the plugin into a temporary
           file.  This may be many gigabytes of data, consuming all space in /var/tmp.  Of course
           this is the whole point of using this filter, but you should be aware of it.

           If using the cache filter, the total size of the cache can be limited (see "CACHE
           MAXIMUM SIZE" in nbdkit-cache-filter(1)).

PARAMETERS

       scan-ahead=false
           By default the filter tries to stay ahead of incoming read requests.  That is to say,
           it starts prefetching at the beginning of the disk and continues incrementally, but if
           the client issues a read beyond the current prefetch point then the filter skips
           forward and begins prefetching after the read.

           However if you set this parameter to false, then this behaviour is disabled.  The
           filter simply prefetches sequentially regardless of client requests.

       scan-clock=false
           By default, if all clients disconnect and then another client connects, prefetching
           resumes at the same place in the disk.  (Like stopping and starting a clock.)

           If you set this parameter to false, then the filter starts prefetching from the
           beginning of the disk again.

       scan-forever=true
           By default the filter scans over the disk once and then stops.

           If you set this parameter to true, then after the disk has been prefetched completely,
           the filter goes back to the beginning and starts over, repeating this for as long as
           nbdkit is running and there are clients connected.

       scan-size=NN
           This parameter controls the prefetch block size.  The default is "2M".  This must be a
           power of 2 and most plugins will have their own limits on the amount of data they can
           prefetch in a single request.

FILES

       $filterdir/nbdkit-scan-filter.so
           The filter.

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

VERSION

       "nbdkit-scan-filter" first appeared in nbdkit 1.32.

SEE ALSO

       nbdkit(1), nbdkit-cache-filter(1), nbdkit-cow-filter(1), nbdkit-file-plugin(1),
       nbdkit-readahead-filter(1), nbdkit-retry-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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