## What is HAMMER?

HAMMER is a file system written for DragonFly that provides instant crash recovery, multi-volume file systems, integrity checking, fine grained history/undo, networked mirroring, and historical snapshots.  HAMMER is the default file system for DragonFly.

### General details
* HAMMER file systems are immediately available after a crash.  There is no fsck.
* A single HAMMER file system can be up to 1 exabyte in size, and can encompass up to 256 volumes, each of which can be up to 4 petabytes (4096 terabytes).
* HAMMER retains a fine-grained history. The state of the filesystem can be accessed live on 30-60 second boundaries without having to make explicit snapshots, up to a configurable fine-grained retention time.
* Coarse-grained history is controlled by snapshots.  By default the system cron generates one snapshot a day and retains 60 days worth.  Snapshots can be accessed live.
* A convenient undo command is provided for single-file history, diffs, and extractions.  Snapshots may be used to access entire directory trees.
* Data and meta-data is CRC-checked for integrity.

### Snapshots
* Snapshots of the file system can be taken at any time, with no limitations.
* Snapshots are indexed by the on-media B-Tree and are extremely storage-efficient.
* Snapshots are "live", and can be accessed at any time.
* Snapshot and historical data retention are controlled through a config file kept in meta-data - no manual maintenance is required for historical files.

### Backups and history
* HAMMER file systems can be split up into multiple pseudo-file systems, or PFSs.  Snapshots and backups can be different for each individual PFS.
* HAMMER PFSs can be backed up continuously or in batch to other HAMMER PFSs, on a per-PFS basis.  
* Backup PFSs (slaves) are functionally identical to the original (master) and can be promoted to a master.
* Slave PFSs can retain file history independent of the master volume's settings.
* HAMMER can efficiently stream bandwidth-controlled near-real-time backup data to slave PFSs on remote hosts.
* 1 master PFS can stream backups to any number of slave PFSs.
* Slave-to-slave mirroring streams are supported, allowing mirrors to be chained together. 
 
For more details, please read the [hammer(5)](http://leaf.dragonflybsd.org/cgi/web-man?command=hammer&section=5) man page.  People interested in porting HAMMER to other operating systems should contact Matthew Dillon at dillon at backplane.com.

## Documentation

[[!table  data="""
Document|Description
[hammer(5)](http://leaf.dragonflybsd.org/cgi/web-man?command=hammer&section=5)|Hammer starter manual page
[hammer(8)](http://leaf.dragonflybsd.org/cgi/web-man?command=hammer&section=8)|Hammer utility manual page
[undo(1)](http://leaf.dragonflybsd.org/cgi/web-man?command=undo&section=1)|Hammer undo command
[hammer.pdf](hammer.pdf)|Hammer major feature document
[hammer](http://leaf.dragonflybsd.org/mailarchive/hammer/)|Hammer mailing list - see [[mailing list|mailinglists]] page
[NYCBSDCon](/presentations/nycbsdcon08)|Slideshow from NYCBSDCon 11 Oct 2008

"""]]

### General Administrative Notes
* ***HAMMER is designed for use on storage media greater than 50G.***  Snapshots and history require significantly different space management than usual, and HAMMER will recover space based on each PFS's pruning schedule.
* By default 60-days worth of snapshots are retained; adjust this based on disk size and activity.  Free space will stabilize once the snapshot limit is reached.  These parameters are programmable.  For example, snapshots are typically turned off on /tmp, /var/tmp, /var/crash, and /usr/obj (each of which is a PFS in a typical DragonFly installation).
* Various cleanup directives can be specified manually via [hammer(5)] for situations which do not fit default expectations.
* The system cron automatically runs HAMMER cleanup functions at least once a day.


More help topics involving HAMMER can be found in the [documentation section](/docs/documentation/) of this website.