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.

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