Provided by: duperemove_0.11.2-3_amd64 bug


       duperemove - Find duplicate extents and print them to stdout


       duperemove [options] files...


       duperemove  is  a  simple  tool  for  finding  duplicated  extents and submitting them for
       deduplication. When given a list of files it will hash the contents of their  extents  and
       compare  those  hashes  to  each  other,  finding and categorizing extents that match each
       other. When given the -d option, duperemove will submit those  extents  for  deduplication
       using the Linux kernel extent-same ioctl.

       duperemove  can store the hashes it computes in a hashfile. If given an existing hashfile,
       duperemove will only compute hashes for those files which have changed since the last run.
       Thus  you  can run duperemove repeatedly on your data as it changes, without having to re-
       checksum unchanged data.  For more on hashfiles see the --hashfile option below as well as
       the Examples section.

       duperemove can also take input from the fdupes program, see the --fdupes option below.


       Duperemove has two major modes of operation one of which is a subset of the other.

   Readonly / Non-deduplicating Mode
       When run without -d (the default) duperemove will print out one or more tables of matching
       extents it has determined would be  ideal  candidates  for  deduplication.  As  a  result,
       readonly mode is useful for seeing what duperemove might do when run with -d.

       Generally,  duperemove  does  not concern itself with the underlying representation of the
       extents it processes. Some of them could be  compressed,  undergoing  I/O,  or  even  have
       already  been deduplicated. In dedupe mode, the kernel handles those details and therefore
       we try not to replicate that work.

   Deduping Mode
       This functions similarly to readonly mode with the exception that the  duplicated  extents
       found  in our "read, hash, and compare" step will actually be submitted for deduplication.
       Extents that have already been deduped will be skipped. An  estimate  of  the  total  data
       deduplicated  will be printed after the operation is complete. This estimate is calculated
       by comparing the total amount of shared bytes in each file before and after the dedupe.


       files can refer to a list of regular files and directories or be a hyphen (-) to read them
       from  standard  input.  If a directory is specified, all regular files within it will also
       be scanned. Duperemove can also be told to recursively  scan  directories  with  the  '-r'

       -r     Enable recursive dir traversal.

       -d     De-dupe the results - only works on btrfs and xfs.

       -A     Opens  files  readonly  when  deduping.  Primarily  for  use by privileged users on
              readonly snapshots.

       -h     Print numbers in human-readable format.

       -q     Quiet mode. Duperemove will only print errors and a short summary of any dedupe.

              Use a file for storage of hashes instead of memory. This option drastically reduces
              the  memory  footprint  of duperemove and is recommended when your data set is more
              than a few files large.  Hashfiles are  also  reusable,  allowing  you  to  further
              reduce the amount of hashing done on subsequent dedupe runs.

              If hashfile does not exist it will be created.  If it exists, duperemove will check
              the file paths stored inside of it for changes.  Files which have changed  will  be
              rescanned  and their updated hashes will be written to the hashfile.  Deleted files
              will be removed from the hashfile.

              New files are only added to the hashfile if they are  discoverable  via  the  files
              argument.   For that reason you probably want to provide the same files list and -r
              arguments on each run of duperemove.  The file discovery algorithm is efficient and
              will only visit each file once, even if it is already in the hashfile.

              Adding a new path to a hashfile is as simple as adding it to the files argument.

              When  deduping from a hashfile, duperemove will avoid deduping files which have not
              changed since the last dedupe.

       -L     Print all files in the hashfile and exit. Requires  the  --hashfile  option.   Will
              print additional information about each file when run with -v.

       -R [file]
              Remove  file from the db and exit. Can be specified multiple times. Duperemove will
              read the list from standard input  if  a  hyphen  (-)  is  provided.  Requires  the
              --hashfile option.

              Note:  If you are piping filenames from another duperemove instance it is advisable
              to do so into a temporary file first as running duperemove  simultaneously  on  the
              same hashfile may corrupt that hashfile.

              Run  in  fdupes  mode.  With  this  option  you  can  pipe  the output of fdupes to
              duperemove to dedupe any duplicate files found. When receiving a file list in  this
              manner, duperemove will skip the hashing phase.

       -v     Be verbose.

              Read data blocks and skip any zeroed blocks, useful for speedup duperemove, but can
              prevent deduplication of zeroed files.

       -b size
              Use the specified block size for reading file extents. Defaults to 128K.

              Use N threads for I/O. This is used by the file hashing and dedupe stages.  Default
              is automatically detected based on number of host cpus.

              Use  N  threads  for  CPU bound tasks. This is used by the duplicate extent finding
              stage. Default is automatically detected based on number of host cpus.

              Note: Hyperthreading can adversely affect performance of the extent finding  stage.
              If  duperemove detects an Intel CPU with hyperthreading it will use half the number
              of cores reported by the system for cpu bound tasks.

              Comma separated list of options which alter how  we  dedupe.  Prepend  'no'  to  an
              option in order to turn it off.

                     Duperemove  can  often  find more dedupe by comparing portions of extents to
                     each other. This can be a lengthy, CPU intensive task so it is turned off by

                     The  code behind this option is under active development and as a result the
                     semantics of the partial argument may change.

                     Defaults to off. Allow dedupe of extents within the same file.

                     Defaults to on. Duperemove uses the fiemap ioctl during the dedupe stage  to
                     optimize  out  already  deduped extents as well as to provide an estimate of
                     the space saved after dedupe operations are complete.

                     Unfortunately, some versions of Btrfs exhibit extremely poor performance  in
                     fiemap  as  the  number  of  references on a file extent goes up. If you are
                     experiencing the dedupe phase slowing down or 'locking up' this  option  may
                     give you a significant amount of performance back.

                     Note:  This  does not turn off all usage of fiemap, to disable fiemap during
                     the file scan stage, you will  also  want  to  use  the  --lookup-extents=no


       --help Prints help text.

              Defaults  to  yes.  Allows  duperemove to skip checksumming some blocks by checking
              their extent state.

              This option is primarily for testing. See the --hashfile option if you want to  use

              Read  hashes  from a hashfile. A file list is not required with this option. Dedupe
              can be done if duperemove is run from the same base directory as is stored  in  the
              hash file (basically duperemove has to be able to find the files).

              This  option is primarily for testing. See the --hashfile option if you want to use

              Write hashes to a hashfile. These can be read in at a later date and deduped from.

              Print debug messages, forces -v if selected.

              Deprecated, see --io-threads above.

              You can choose between murmur3 and xxhash. The default is murmur3  as  it  is  very
              fast  and can generate 128 bit digests for a very small chance of collision. Xxhash
              may be faster but generates only 64 bit digests. Both hashes are fast  enough  that
              the default should work well for the overwhelming majority of users.

              You an exclude certain files and folders from the deduplication process. This might
              be benefical for skipping subvolume snapshot mounts,  for  instance.  You  need  to
              provide  full  path  for  exclusion.  For example providing just a file name with a
              wildcard i.e duperemove  --exclude  file-*  won't  ever  match  because  internally
              duperemove  works with absolute paths. Another thing to keep in mind is that shells
              usually expand glob pattern so the passed in  pattern  ought  to  also  be  quoted.
              Taking  everything  into consideration the correct way to pass an exclusion pattern
              is duperemove --exclude "/path/to/dir/file*" /path/to/dir


   Simple Usage
       Dedupe the files in directory /foo, recurse into all subdirectories. You only want to  use
       this for small data sets.

              duperemove -dr /foo

       Use duperemove with fdupes to dedupe identical files below directory foo.

              fdupes -r /foo | duperemove --fdupes

   Using Hashfiles
       Duperemove can optionally store the hashes it calculates in a hashfile. Hashfiles have two
       primary advantages - memory usage and re-usability. When using a hashfile, duperemove will
       stream computed hashes to it, instead of main memory.

       If  Duperemove  is run with an existing hashfile, it will only scan those files which have
       changed since the last time the hashfile was updated. The files  argument  controls  which
       directories  duperemove  will scan for newly added files. In the simplest usage, you rerun
       duperemove with the same parameters and it will only scan changed or newly added  files  -
       see the first example below.

       Dedupe  the  files  in  directory foo, storing hashes in foo.hash. We can run this command
       multiple times and duperemove will only checksum and dedupe changed or newly added files.

              duperemove -dr --hashfile=foo.hash foo/

       Don't scan for new files, only update changed or deleted files, then dedupe.

              duperemove -dr --hashfile=foo.hash

       Add directory bar to our hashfile and discover any files that were recently added to foo.

              duperemove -dr --hashfile=foo.hash foo/ bar/

       List the files tracked by foo.hash.

              duperemove -L --hashfile=foo.hash


   Is there an upper limit to the amount of data duperemove can process?
       Duperemove v0.11 is fast at reading and  cataloging  data.  Dedupe  runs  will  be  memory
       limited  unless  the  '--hashfile'  option  is  used.  '--hashfile'  allows  duperemove to
       temporarily store duplicated hashes to disk, thus removing the large memory  overhead  and
       allowing  for  a far larger amount of data to be scanned and deduped. Realistically though
       you will be limited by the speed  of  your  disks  and  cpu.  In  those  situations  where
       resources  are limited you may have success by breaking up the input data set into smaller

       When using a hashfile, duperemove will only  store  duplicate  hashes  in  memory.  During
       normal  operation then the hash tree will make up the largest portion of duperemove memory
       usage. As of Duperemove v0.11 hash entries are 88 bytes in size. If you know the number of
       duplicate  blocks  in  your  data set you can get a rough approximation of memory usage by
       multiplying with the hash entry size.

       Actual performance numbers are dependent on hardware - up to date testing  information  is
       kept on the duperemove wiki (see below for the link).

   How large of a hashfile will duperemove create?
       Hashfiles are essentially sqlite3 database files with several tables, the largest of which
       are the files and extents tables. Each extents table entry is about 72 bytes  though  that
       may  grow  as features are added. The size of a files table entry depends on the file path
       but a good estimate is around 270 bytes per file. The number of extents in a data  set  is
       directly proportional to file fragmentation level.

       If  you know the total number of extents and files in your data set then you can calculate
       the hashfile size as:

       Hashfile Size = Num Hashes X 72 + Num Files X 270

       Using a real world example of 1TB (8388608 128K blocks) of data over 1000 files:

       8388608 * 72 + 270 * 1000 = 755244720 or about 720MB for 1TB spread over 1000 files.

       Note that none of this takes database overhead into account.

   Is is safe to interrupt the program (Ctrl-C)?
       Yes, Duperemove uses a transactional database engine and  organizes  db  changes  to  take
       advantage  of  those features. The result is that you should be able to ctrl-c the program
       at any point and re-run without experiencing corruption of your hashfile.

   I got two identical files, why are they not deduped?
       Duperemove by default works on extent granularity. What this means is  if  there  are  two
       files  which are logically identical (have the same content) but are laid out on disk with
       different extent structure they won't be deduped. For example if 2 files are 128k each and
       their content are identical but one of them consists of a single 128k extent and the other
       of 2 x 64k extents then they won't be deduped. This behavior is dependent on  the  current
       implementation and is subject to change as duperemove is being improved.

   How can I find out my space savings after a dedupe?
       Duperemove will print out an estimate of the saved space after a dedupe operation for you.

       You  can  get  a  more  accurate  picture  by  running 'btrfs fi df' before and after each
       duperemove run.

       Be careful about using the 'df' tool on btrfs - it is common for  space  reporting  to  be
       'behind'  while delayed updates get processed, so an immediate df after deduping might not
       show any savings.

   Why is the total deduped data report an estimate?
       At the moment duperemove can detect that some underlying extents  are  shared  with  other
       files, but it can not resolve which files those extents are shared with.

       Imagine  duperemove  is examing a series of files and it notes a shared data region in one
       of them. That data could be shared with a file outside of  the  series.  Since  duperemove
       can't  resolve  that  information  it  will  account  the  shared  data against our dedupe
       operation while in reality, the kernel might deduplicate it further for us.

   Why are my files showing dedupe but my disk space is not shrinking?
       This is a little complicated, but it comes down to a feature in Btrfs called _bookending_.
       The Btrfs wiki explains this in detail:

       Essentially  though,  the underlying representation of an extent in Btrfs can not be split
       (with small exception). So sometimes we can end up in a situation where a file extent gets
       partially deduped (and the extents marked as shared) but the underlying extent item is not
       freed or truncated.

   Is duperemove safe for my data?
       Yes. To be specific, duperemove does not deduplicate the data  itself.   It  simply  finds
       candidates  for dedupe and submits them to the Linux kernel extent-same ioctl. In order to
       ensure data integrity, the kernel locks out other access to the file and does  a  byte-by-
       byte compare before proceeding with the dedupe.

   What is the cost of deduplication?
       Deduplication  will  lead  to  increased  fragmentation.  The blocksize chosen can have an
       effect on this. Larger blocksizes will fragment less but may not save you as  much  space.
       Conversely,   smaller   block  sizes  may  save  more  space  at  the  cost  of  increased


       Deduplication is currently only supported by the btrfs and xfs filesystem.

       The Duperemove project page can be found at

       There is also a wiki at


       hashstats(8) filesystems(5) btrfs(8) xfs(8) fdupes(1)