1=head1 NAME 2 3nbdkit-readahead-filter - prefetch data when reading sequentially 4 5=head1 SYNOPSIS 6 7 nbdkit --filter=readahead plugin 8 9=head1 DESCRIPTION 10 11C<nbdkit-readahead-filter> is a filter that prefetches data when the 12client is reading sequentially. 13 14A common use for this filter is to accelerate sequential copy 15operations (like S<C<qemu-img convert>>) when plugin requests have a 16high overhead (like L<nbdkit-curl-plugin(1)>). For example: 17 18 nbdkit -U - --filter=readahead curl https://example.com/disk.img \ 19 --run 'qemu-img convert $nbd disk.img' 20 21The filter uses a simple adaptive algorithm which accelerates 22sequential reads, but has a small penalty if the client does random 23reads. If the client mixes reads with writes or write-like operations 24(trimming, zeroing) then it will work but there can be a large 25performance penalty. 26 27=head1 PARAMETERS 28 29There are no parameters specific to nbdkit-readahead-filter. Any 30parameters are passed through to and processed by the underlying 31plugin in the normal way. 32 33=head1 FILES 34 35=over 4 36 37=item F<$filterdir/nbdkit-readahead-filter.so> 38 39The filter. 40 41Use C<nbdkit --dump-config> to find the location of C<$filterdir>. 42 43=back 44 45=head1 VERSION 46 47C<nbdkit-readahead-filter> first appeared in nbdkit 1.12. 48 49=head1 SEE ALSO 50 51L<nbdkit(1)>, 52L<nbdkit-cache-filter(1)>, 53L<nbdkit-curl-plugin(1)>, 54L<nbdkit-retry-filter(1)>, 55L<nbdkit-ssh-plugin(1)>, 56L<nbdkit-vddk-plugin(1)>, 57L<nbdkit-filter(3)>, 58L<qemu-img(1)>. 59 60=head1 AUTHORS 61 62Richard W.M. Jones 63 64=head1 COPYRIGHT 65 66Copyright (C) 2019 Red Hat Inc. 67