3 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.
6 * HAMMER file systems are immediately available after a crash, and do not require fsck.
7 * 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.
8 * Historical views of all files on the system are saved on each sync to disk.
11 * Snapshots of the file system can be taken at any time and in any amount.
12 * Snapshots take only the space needed to describe changes to disk, not the actual size of the volume.
13 * Snapshots are "live", and can be accessed at any time.
14 * Snapshot and historical data retention are controlled through a config file kept in meta-data - no manual maintenance is required for historical files.
16 ### Backups and history
17 * HAMMER file systems can be split up into multiple pseudo-file systems, or PFS.
18 * HAMMER volumes can be continually backed up to other HAMMER volumes on a per-PFS basis.
19 * Backup volumes (slaves) are functionally identical to the original (master) and can be promoted to a master volume.
20 * Slave PFSs can retain file history independent of the master volume, saving more or less snapshots as desired.
21 * Backups can be on a local network or over the Internet. No minimum data speed required; the backup will catch up as bandwidth allows.
22 * Data is CRC-checked for integrity.
24 **Note: HAMMER should be used on 50G or larger disks.**
26 For more details, please read the [hammer(5)](http://leaf.dragonflybsd.org/cgi/web-man?command=hammer§ion=5) man page. People interested in porting HAMMER to other operating systems should contact Matthew Dillon at dillon at backplane.com.
32 [hammer(5)](http://leaf.dragonflybsd.org/cgi/web-man?command=hammer§ion=5)|Hammer starter manual page
33 [hammer(8)](http://leaf.dragonflybsd.org/cgi/web-man?command=hammer§ion=8)|Hammer utility manual page
34 [undo(1)](http://leaf.dragonflybsd.org/cgi/web-man?command=undo§ion=1)|Hammer undo command
35 [hammer.pdf](hammer.pdf)|Hammer major feature document
36 [hammer](http://leaf.dragonflybsd.org/mailarchive/hammer/)|Hammer mailing list - see [[mailing list|mailinglists]] page
37 [NYCBSDCon](/presentations/nycbsdcon08)|Slideshow from NYCBSDCon 11 Oct 2008
41 More help topics involving HAMMER can be found in the [documentation section](/docs/documentation/) of this website.