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.
- 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 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) man page. People interested in porting HAMMER to other operating systems should contact Matthew Dillon at dillon at backplane.com.
|hammer(5)||Hammer starter manual page|
|hammer(8)||Hammer utility manual page|
|undo(1)||Hammer undo command|
|hammer.pdf||Hammer major feature document|
|hammer||Hammer mailing list - see mailing list page|
|NYCBSDCon||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 of this website.