1nbdkit-readahead-filter(1)          NBDKIT          nbdkit-readahead-filter(1)
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NAME

6       nbdkit-readahead-filter - prefetch data when reading sequentially
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SYNOPSIS

9        nbdkit --filter=readahead plugin
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DESCRIPTION

12       "nbdkit-readahead-filter" is a filter that prefetches data when the
13       client is reading sequentially.
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15       A common use for this filter is to accelerate sequential copy
16       operations (like "qemu-img convert") when plugin requests have a high
17       overhead (like nbdkit-curl-plugin(1)).  For example:
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19        nbdkit -U - --filter=readahead curl https://example.com/disk.img \
20               --run 'qemu-img convert $nbd disk.img'
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22       The filter uses a simple adaptive algorithm which accelerates
23       sequential reads, but has a small penalty if the client does random
24       reads.  If the client mixes reads with writes or write-like operations
25       (trimming, zeroing) then it will work but there can be a large
26       performance penalty.
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PARAMETERS

29       There are no parameters specific to nbdkit-readahead-filter.  Any
30       parameters are passed through to and processed by the underlying plugin
31       in the normal way.
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FILES

34       $filterdir/nbdkit-readahead-filter.so
35           The filter.
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37           Use "nbdkit --dump-config" to find the location of $filterdir.
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VERSION

40       "nbdkit-readahead-filter" first appeared in nbdkit 1.12.
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SEE ALSO

43       nbdkit(1), nbdkit-cache-filter(1), nbdkit-curl-plugin(1),
44       nbdkit-retry-filter(1), nbdkit-ssh-plugin(1), nbdkit-vddk-plugin(1),
45       nbdkit-filter(3), qemu-img(1).
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AUTHORS

48       Richard W.M. Jones
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51       Copyright (C) 2019 Red Hat Inc.
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LICENSE

54       Redistribution and use in source and binary forms, with or without
55       modification, are permitted provided that the following conditions are
56       met:
57
58       ·   Redistributions of source code must retain the above copyright
59           notice, this list of conditions and the following disclaimer.
60
61       ·   Redistributions in binary form must reproduce the above copyright
62           notice, this list of conditions and the following disclaimer in the
63           documentation and/or other materials provided with the
64           distribution.
65
66       ·   Neither the name of Red Hat nor the names of its contributors may
67           be used to endorse or promote products derived from this software
68           without specific prior written permission.
69
70       THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
71       EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
72       IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
73       PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
74       LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
75       CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
76       SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
77       BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
78       WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
79       OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
80       ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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84nbdkit-1.18.4                     2020-04-16        nbdkit-readahead-filter(1)
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