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Easy Incremental Backups With HAMMER Filesystem

Finally I’ve made some progress in one of my miniprojects! At least an initial version of the hammer-backup script has been pushed to the master branch upstream in the DragonFly BSD repository, see commitdiff here.

For those who don’t know, DragonFly BSD is a fork of FreeBSD 4.x. Its leader, Matthew Dillon, implemented a log structured filesystem from scratch called HAMMER. It’s divided into independent “sub filesystems” called PFS (pseudo-filesystem).

DragonFly BSD standard installation on a HAMMER filesystem provides a PFS for every typical UNIX system directory (/usr, /var), see below:

koprulu# df -h
Filesystem                                               Size   Used  Avail Capacity  Mounted on
ROOT                                                     926G    14G   912G     2%    /
devfs                                                    1.0K   1.0K     0B   100%    /dev
/dev/part-by-uuid/cb4561e2-8aaf-11e3-9867-535400123456   1.2G   303M   784M    28%    /boot
/pfs/@@-1:00001                                          926G    14G   912G     2%    /var
/pfs/@@-1:00002                                          926G    14G   912G     2%    /tmp
/pfs/@@-1:00003                                          926G    14G   912G     2%    /usr
/pfs/@@-1:00004                                          926G    14G   912G     2%    /home
/pfs/@@-1:00005                                          926G    14G   912G     2%    /usr/obj
/pfs/@@-1:00006                                          926G    14G   912G     2%    /var/crash
/pfs/@@-1:00007                                          926G    14G   912G     2%    /var/tmp
procfs                                                   4.0K   4.0K     0B   100%    /proc

HAMMER is a copy-on-write filesystem that records history of every change on it. It’s also transactional and every transaction has a transaction ID associated (TID). Here’s where things get really interesting, HAMMER provides a tool to operate the filesystem itself called ‘hammer’.

It can dump the contents of every PFS to stdout and that’s what the hammer-script uses.

You can see how the script itself works here

Migrated to Octopress!

I’ve migrated all the contents of my old fashioned static page to Octopress! Keeping the old design for historical purposes, see below :-)