Ubuntu Manpage: nbdkit-blocksize-filter

Provided by: nbdkit_1.36.3-1ubuntu10_amd64

NAME

       nbdkit-blocksize-filter - nbdkit blocksize filter

SYNOPSIS

        nbdkit --filter=blocksize plugin [minblock=SIZE] [maxdata=SIZE] \
            [maxlen=SIZE] [plugin-args...]

DESCRIPTION

       "nbdkit-blocksize-filter" is a filter that ensures various block size limits are met on transactions
       presented to the plugin.  The NBD protocol permits clients to send requests with a granularity as small
       as 1 byte or as large as nearly 4 gigabytes, although it suggests that portable clients should align
       requests to 512 bytes and not exceed 32 megabytes without prior coordination with the server.

       Meanwhile, some plugins require requests to be aligned to 512-byte multiples, or may enforce a maximum
       transaction size to bound the time or memory resources spent by any one command (note that nbdkit itself
       refuses a read or write larger than 64 megabytes, while many other NBD servers limit things to 32
       megabytes).  The blocksize filter can be used to modify the client requests to meet the plugin
       restrictions.

       This filter can be combined with nbdkit-blocksize-policy-filter(1) to advertise different block sizes to
       the client.

PARAMETERS

The nbdkit-blocksize-filter accepts the following parameters.

minblock=SIZE
The minimum block size and alignment to pass to the plugin. This must be a power of two, and no
larger than 64k. If omitted, this defaults to the minimum block size of the underlying plugin, or 1
if the plugin did not report a minimum block size. The filter rounds up read requests to alignment
boundaries, performs read-modify-write cycles for any unaligned head or tail of a write or zero
request, and silently ignores any unaligned head or tail of a trim request. The filter also
truncates the plugin size down to an aligned value (as it cannot safely operate on the unaligned
tail). If you need to round the image size up instead to access the last few bytes, combine this
filter with nbdkit-truncate-filter(1).

This parameter understands the suffix 'k' for 1024.

maxdata=SIZE
The maximum block size for any single transaction with data (read and write). If omitted, this
defaults to the minimum of 64 megabytes (that is, the nbdkit maximum) or any maximum reported by the
underlying plugin. This need not be a power of two, but must be an integer multiple of "minblock".
The filter fragments any larger client request into multiple plugin requests.

This parameter understands the suffixes 'k', 'M', and 'G' for powers of 1024.

maxlen=SIZE
The maximum length for any single transaction without data (trim, zero, cache or extents). If
omitted, this defaults to 0xffffffff rounded down to "minblock" alignment (that is, the inherent
32-bit limit of the NBD protocol). This need not be a power of two, but must be an integer multiple
of "minblock", and should be at least as large as "maxdata". The filter fragments any larger client
request into multiple plugin requests.

This parameter understands the suffixes 'k', 'M', and 'G' for powers of 1024.

EXAMPLES

       Allow an arbitrary client to use the VDDK plugin (which is limited to 512-byte blocks), without having to
       fix the client to avoid sending unaligned requests:

        nbdkit --filter=blocksize vddk minblock=512 file=/absolute/path/to/file.vmdk

       Allow an arbitrary client tuned to nbdkit's 64 megabyte sizing to connect to a remote server that insists
       on 32 megabyte sizing, via the nbd plugin:

        nbdkit --filter=blocksize nbd maxdata=32M socket=/path/to/socket

       Serve  a  file  as if it were a block device that insists on 4k alignment, while still allowing access to
       any unaligned bytes at the end of the file:

        nbdkit --filter=blocksize --filter=truncate file /path/to/file \
        minblock=4k round-up=4k

FILES

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

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

VERSION

       "nbdkit-blocksize-filter" first appeared in nbdkit 1.4.

AUTHORS

       Eric Blake

COPYRIGHT

       Copyright Red Hat

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
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       OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
       DAMAGE.

nbdkit-1.36.3                                      2024-03-31                         nbdkit-blocksize-filter(1)