## 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, networked mirroring, and historical snapshots.  HAMMER is the default file system for DragonFly.

### General details
* HAMMER file systems are immediately available after a crash, and do not require fsck.
* A single HAMMER file system can be up to 1 exabyte in size, and can encompass up to 256 volumes, of which each can be up to 4 petabytes.
* Historical views of all files on the system are saved on each sync to disk.

### Snapshots
* Snapshots of the file system can be taken at any time and in any amount.
* Snapshots take only the space needed to describe changes to disk, not the actual size of the volume.
* 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 PFS.
* HAMMER volumes can be continually backed up to other HAMMER volumes on a per-PFS basis.  
* Backup volumes (slaves) are functionally identical to the original (master) and can be promoted to a master volume.
* Slave PFSs can retain file history independent of the master volume, saving more or less snapshots as desired.
* Backups can be on a local network or over the Internet.  No minimum data speed required; the backup will catch up as bandwidth allows.
* Data is CRC-checked for integrity. 

**Note: HAMMER should be used on 50G or larger disks.** 

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

"""]]

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