-# The HAMMER Filesystem
-
## 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.
+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, 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.
+* 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 and in any amount.
-* Snapshots take only the space needed to describe changes to disk, not the actual size of the volume.
+* 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 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.**
-
+* 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§ion=5) man page. People interested in porting HAMMER to other operating systems should contact Matthew Dillon at dillon at backplane.com.
## Documentation
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+### 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.
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