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HAMMER(5) DragonFly File Formats Manual HAMMER(5)
HAMMER - HAMMER file system
To compile this driver into the kernel, place the following line in your
kernel configuration file:
Alternatively, to load the driver as a module at boot time, place the
following line in loader.conf(5):
To mount via fstab(5):
/dev/ad0s1d[:/dev/ad1s1d:...] /mnt hammer rw 2 0
The HAMMER file system provides facilities to store file system data onto
disk devices and is intended to replace ffs(5) as the default file system
Among its features are instant crash recovery, large file systems
spanning multiple volumes, data integrity checking, data deduplication,
fine grained history retention and snapshots, pseudo-filesystems (PFSs),
mirroring capability and unlimited number of files and links.
All functions related to managing HAMMER file systems are provided by the
newfs_hammer(8), mount_hammer(8), hammer(8), sysctl(8), chflags(1), and
For a more detailed introduction refer to the paper and slides listed in
the SEE ALSO section. For some common usages of HAMMER see the EXAMPLES
Description of HAMMER features:
Instant Crash Recovery
After a non-graceful system shutdown, HAMMER file systems will be brought
back into a fully coherent state when mounting the file system, usually
within a few seconds.
In the unlikely case HAMMER mount fails due redo recovery (stage 2
recovery) being corrupted, a workaround to skip this stage can be applied
by setting the following tunable:
Possible values are:
0 Run redo recovery normally and fail to mount in the case of error
1 Run redo recovery but continue mounting if an error appears.
2 Completely bypass redo recovery.
Related commands: mount_hammer(8)
Large File Systems & Multi Volume
A HAMMER file system can be up to 1 Exabyte in size. It can span up to
256 volumes, each volume occupies a DragonFly disk slice or partition, or
another special file, and can be up to 4096 TB in size. Minimum
recommended HAMMER file system size is 50 GB. For volumes over 2 TB in
size gpt(8) and disklabel64(8) normally need to be used.
Related hammer(8) commands: volume-add, volume-del, volume-list,
volume-blkdevs; see also newfs_hammer(8)
Data Integrity Checking
HAMMER has high focus on data integrity, CRC checks are made for all
major structures and data. HAMMER snapshots implements features to make
data integrity checking easier: The atime and mtime fields are locked to
the ctime for files accessed via a snapshot. The st_dev field is based
on the PFS shared-uuid and not on any real device. This means that
archiving the contents of a snapshot with e.g. tar(1) and piping it to
something like md5(1) will yield a consistent result. The consistency is
also retained on mirroring targets.
To save disk space data deduplication can be used. Data deduplication
will identify data blocks which occur multiple times and only store one
copy, multiple reference will be made to this copy.
Related hammer(8) commands: dedup, dedup-simulate, cleanup, config
The HAMMER file system uses 64-bit transaction ids to refer to historical
file or directory data. Transaction ids used by HAMMER are monotonically
increasing over time. In other words: when a transaction is made, HAMMER
will always use higher transaction ids for following transactions. A
transaction id is given in hexadecimal format 0x016llx, such as
Related hammer(8) commands: snapshot, snap, snaplo, snapq, snapls,
History & Snapshots
History metadata on the media is written with every sync operation, so
that by default the resolution of a file's history is 30-60 seconds until
the next prune operation. Prior versions of files and directories are
generally accessible by appending `@@' and a transaction id to the name.
The common way of accessing history, however, is by taking snapshots.
Snapshots are softlinks to prior versions of directories and their files.
Their data will be retained across prune operations for as long as the
softlink exists. Removing the softlink enables the file system to
reclaim the space again upon the next prune & reblock operations. In
HAMMER Version 3+ snapshots are also maintained as file system meta-data.
Related hammer(8) commands: cleanup, history, snapshot, snap, snaplo,
snapq, snaprm, snapls, config, viconfig; see also undo(1)
Pruning & Reblocking
Pruning is the act of deleting file system history. By default only
history used by the given snapshots and history from after the latest
snapshot will be retained. By setting the per PFS parameter prune-min,
history is guaranteed to be saved at least this time interval. All other
history is deleted. Reblocking will reorder all elements and thus
defragment the file system and free space for reuse. After pruning a
file system must be reblocked to recover all available space. Reblocking
is needed even when using the nohistory mount_hammer(8) option or
Related hammer(8) commands: cleanup, snapshot, prune, prune-everything,
rebalance, reblock, reblock-btree, reblock-inodes, reblock-dirs,
A pseudo-filesystem, PFS for short, is a sub file system in a HAMMER file
system. All disk space in a HAMMER file system is shared between all
PFSs in it, so each PFS is free to use all remaining space. A HAMMER
file system supports up to 65536 PFSs. The root of a HAMMER file system
is PFS# 0, it is called the root PFS and is always a master PFS.
A non-root PFS can be either master or slave. Slaves are always read-
only, so they can't be updated by normal file operations, only by
hammer(8) operations like mirroring and pruning. Upgrading slaves to
masters and downgrading masters to slaves are supported.
It is recommended to use a null mount to access a PFS, except for root
PFS; this way no tools are confused by the PFS root being a symlink and
inodes not being unique across a HAMMER file system.
Many hammer(8) operations operates per PFS, this includes mirroring,
offline deduping, pruning, reblocking and rebalancing.
Related hammer(8) commands: pfs-master, pfs-slave, pfs-status,
pfs-update, pfs-destroy, pfs-upgrade, pfs-downgrade; see also
Mirroring is copying of all data in a file system, including snapshots
and other historical data. In order to allow inode numbers to be
duplicated on the slaves HAMMER mirroring feature uses PFSs. A master or
slave PFS can be mirrored to a slave PFS. I.e. for mirroring multiple
slaves per master are supported, but multiple masters per slave are not.
HAMMER does not support multi-master clustering and mirroring.
Related hammer(8) commands: mirror-copy, mirror-stream, mirror-read,
mirror-read-stream, mirror-write, mirror-dump
Fsync Flush Modes
The HAMMER file system implements several different fsync() flush modes,
the mode used is set via the vfs.hammer.flush_mode sysctl, see hammer(8)
Unlimited Number of Files and Links
There is no limit on the number of files or links in a HAMMER file
system, apart from available disk space.
HAMMER file systems support NFS export. NFS export of PFSs is done using
null mounts (for file/directory in root PFS null mount is not needed).
For example, to export the PFS /hammer/pfs/data, create a null mount,
e.g. to /hammer/data and export the latter path.
Don't export a directory containing a PFS (e.g. /hammer/pfs above). Only
null mount for PFS root (e.g. /hammer/data above) should be exported
(subdirectory may be escaped if exported).
File System Versions
As new features have been introduced to HAMMER a version number has been
bumped. Each HAMMER file system has a version, which can be upgraded to
support new features.
Related hammer(8) commands: version, version-upgrade; see also
Preparing the File System
To create and mount a HAMMER file system use the newfs_hammer(8) and
mount_hammer(8) commands. Note that all HAMMER file systems must have a
unique name on a per-machine basis.
newfs_hammer -L HOME /dev/ad0s1d
mount_hammer /dev/ad0s1d /home
Similarly, multi volume file systems can be created and mounted by
specifying additional arguments.
newfs_hammer -L MULTIHOME /dev/ad0s1d /dev/ad1s1d
mount_hammer /dev/ad0s1d /dev/ad1s1d /home
Once created and mounted, HAMMER file systems need periodic clean up
making snapshots, pruning and reblocking, in order to have access to
history and file system not to fill up. For this it is recommended to
use the hammer(8) cleanup metacommand.
By default, DragonFly is set up to run hammer cleanup nightly via
It is also possible to perform these operations individually via
crontab(5). For example, to reblock the /home file system every night at
2:15 for up to 5 minutes:
15 2 * * * hammer -c /var/run/HOME.reblock -t 300 reblock /home \
The hammer(8) utility's snapshot command provides several ways of taking
snapshots. They all assume a directory where snapshots are kept.
hammer snapshot /home /snaps/snap1
(...after some changes in /home...)
hammer snapshot /home /snaps/snap2
The softlinks in /snaps point to the state of the /home directory at the
time each snapshot was taken, and could now be used to copy the data
somewhere else for backup purposes.
By default, DragonFly is set up to create nightly snapshots of all HAMMER
file systems via periodic(8) and to keep them for 60 days.
A snapshot directory is also the argument to the hammer(8) prune command
which frees historical data from the file system that is not pointed to
by any snapshot link and is not from after the latest snapshot and is
older than prune-min.
hammer prune /snaps
Mirroring is set up using HAMMER pseudo-filesystems (PFSs). To associate
the slave with the master its shared UUID should be set to the master's
shared UUID as output by the hammer pfs-master command.
hammer pfs-master /home/pfs/master
hammer pfs-slave /home/pfs/slave shared-uuid=<master's shared uuid>
The /home/pfs/slave link is unusable for as long as no mirroring
operation has taken place.
To mirror the master's data, either pipe a mirror-read command into a
mirror-write or, as a short-cut, use the mirror-copy command (which works
across a ssh(1) connection as well). Initial mirroring operation has to
be done to the PFS path (as mount_null(8) can't access it yet).
hammer mirror-copy /home/pfs/master /home/pfs/slave
It is also possible to have the target PFS auto created by just issuing
the same mirror-copy command, if the target PFS doesn't exist you will be
prompted if you would like to create it. You can even omit the prompting
by using the -y flag:
hammer -y mirror-copy /home/pfs/master /home/pfs/slave
After this initial step null mount can be setup for /home/pfs/slave.
Further operations can use null mounts.
mount_null /home/pfs/master /home/master
mount_null /home/pfs/slave /home/slave
hammer mirror-copy /home/master /home/slave
To NFS export from the HAMMER file system /hammer the directory
/hammer/non-pfs without PFSs, and the PFS /hammer/pfs/data, the latter is
null mounted to /hammer/data.
Add to /etc/fstab (see fstab(5)):
/hammer/pfs/data /hammer/data null rw
Add to /etc/exports (see exports(5)):
hammer: System has insuffient buffers to rebalance the tree. nbuf <
%d Rebalancing a HAMMER PFS uses quite a bit of memory and can't be done
on low memory systems. It has been reported to fail on 512MB systems.
Rebalancing isn't critical for HAMMER file system operation; it is done
by hammer rebalance, often as part of hammer cleanup.
chflags(1), md5(1), tar(1), undo(1), exports(5), ffs(5), fstab(5),
disklabel64(8), gpt(8), hammer(8), mount_hammer(8), mount_null(8),
newfs_hammer(8), periodic(8), sysctl(8)
Matthew Dillon, The HAMMER Filesystem,
http://www.dragonflybsd.org/hammer/hammer.pdf, June 2008.
Matthew Dillon, Slideshow from NYCBSDCon 2008,
http://www.dragonflybsd.org/presentations/nycbsdcon08/, October 2008.
Michael Neumann, Slideshow for a presentation held at KIT
(http://www.kit.edu), http://www.ntecs.de/talks/HAMMER.pdf, January 2010.
The HAMMER file system has a front-end which processes VNOPS and issues
necessary block reads from disk, and a back-end which handles meta-data
updates on-media and performs all meta-data write operations. Bulk file
write operations are handled by the front-end. Because HAMMER defers
meta-data updates virtually no meta-data read operations will be issued
by the frontend while writing large amounts of data to the file system or
even when creating new files or directories, and even though the kernel
prioritizes reads over writes the fact that writes are cached by the
drive itself tends to lead to excessive priority given to writes.
There are four bioq sysctls, shown below with default values, which can
be adjusted to give reads a higher priority:
If a higher read priority is desired it is recommended that the
kern.bioq_reorder_minor_interval be increased to 15, 30, or even 60, and
the kern.bioq_reorder_burst_bytes be decreased to 262144 or 524288.
The HAMMER file system first appeared in DragonFly 1.11.
The HAMMER file system was designed and implemented by Matthew Dillon
<email@example.com>, data deduplication was added by Ilya Dryomov.
This manual page was written by Sascha Wildner and updated by Thomas
DragonFly 5.9-DEVELOPMENT July 7, 2017 DragonFly 5.9-DEVELOPMENT