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BLKDISCARD(8)               System Manager's Manual              BLKDISCARD(8)

     blkdiscard - tool to discard portions of a device or file

     blkdiscard [-h] [-l length] [-p chunksize] [-n] [-o firstbyte] [-R] [-s]
                [-V] [-v] [-z] file
     fdiscard [-f firstbyte] [-h] [-l length] [-m chunksize] [-n] [-R] [-s]
              [-V] [-v] [-z] file

     The blkdiscard program can be used to discard portions of storage, and
     works on either device files directly or upon files inside of a mounted
     file-system.  It provides access to the fdiscard(2) system call, and
     relies upon the backing device to have support in both the blkdiscard
     driver and in the hardware itself.

     The fdiscard interface defaults to the "no-run" mode and requires the -R
     option to perform any actions.

     Available options:

     -f firstbyte  Set the first byte to be discarded.  The default is 0.

     -h            Display built-in help.

     -l length     Limit the length to be discarded.  The default is the full
                   device or file size, or the rest of the device or file when
                   the start offset has been set.

     -m chunksize  Set the per-operation chunksize.  The default is 32 MiB.

     -n            Set the "no-run" option, and do not actually call
                   fdiscard(2), secure erase, or write zeroes to the file.

     -o firstbyte  This flag is an alias for the -f flag.

     -p chunksize  This flag is an alias for the -m flag.

     -R            Turn off the "no-run mode."

     -s            Use secure erase operation.  This requires specific
                   hardware support and is not currently supported.

     -V            Display the program version and exit.

     -v            Set the verbose flag.

     -z            Write zeroes instead of calling fdiscard(2).

     To have a dry-run cleaning of any particular file or device:

           # blkdiscard -n -v filename
           # blkdiscard -n -v /dev/rwd0b

     To discard all the blocks on a swap device:

           # blkdiscard -v /dev/rwd0b

     To discard all but the first 256MiB of a device, using 128MiB increments:

           # blkdiscard -v -f 256m -m 128m /dev/rwd0c

     To zero out the contents of a file or device:

           # blkdiscard -v -z filename
           # blkdiscard -v -z /dev/device


     The blkdiscard command first appeared linux-util.  This implementation
     was originally called fdiscard-stuff and has been converted to be
     blkdiscard(8) compatible and appeared in NetBSD 10.0.

     The blkdiscard command was written by Matthew R. Green

     The secure erase funtionality of the -s option is not yet implemented.

NetBSD 9.99                    February 6, 2022                    NetBSD 9.99