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-torrent-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:

• 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.