- instant crash recovery
- large file systems and multivolume support
- data integrity checking
- history and snapshots
- reblocking (on the fly defragmentation)
Let's mount in under /mnt directory by using "noatime" option. mount_hammer command must be used for this operation.
Looking to df output you can see our new hammer filesystem labeled as "mydata". Now, let's see where hammer filesystem differs from ffs.
History metadata of a hammer filesystem is written on every 30 seconds. You can use hammer utility to access your files history data. Usage is simple as, calling hammer utility with history command and given your file name which you want to see whole history.
# hammer history /mnt/test.txt
You may be noticed that I have used "sync" command, because I didn't want to wait the the kernel sync operation occur (30 seconds) and forced it by using sync comand to get history result fast.
Now let's see hammer's snapshot feature. I have created a directory called "/snapshots" where I want to keep all of my hammer filesystem snaphosts. We'll use hammer utility for taking snapshots.
# hammer snapshot /mnt /snapshots/mnt_snapshot1
I have created a file on /mnt filesystem and took a snapshot. After a sucsessful snapshot, I deleted the original file from /mnt and accessed it by using the snapshot I took before deleting the file (/snapshots/mnt_snapshot1/test.txt).
You can see and rollback every change between the taken snapshots and original files by using hammer's undo command. You can use "-d" option to get diff output to see every changes on the given file line by line between every snapshot and original one. Also, "-a" option gives you whole change history of given file. Here, I created a test file called "myfile.txt" under my new hammer filesystem (/mnt) and then took a snapshot. After taking snapshot, I added one more line to my test file and used "undo -d /mnt/myfile.txt" to get diff output and see what changes made the file between snapshots.
As I mentioned above, you can use multi volumes and create a big disk volume with hammer filesystem. Now, we'll create a single hammer volume by using two disks (ad1 and ad3).
You just call newfs_hammer and mount_hammer commands by only giving two or more disk device names instead of one.Commands are straight forward. First, we need to create new hammer filesystem on disks.
# newfs_hammer -L big /dev/ad1 /dev/ad3
I have used to disks and each size is 8GB. After creating new hammer filesystems on disks , we can mount them as one big volume. (I know 8GB is not big :) ).
# mount_hammer /dev/ad1 /dev/ad3 /mnt
You can see on /mnt mount point we have a disk in size 16GB, total of two 8GB disks.
Hammer uses Pseudo File Systems (PFS) to duplicate inode numbers to slaves. Therefore, mount_null command required for this operation. This duplication is triggered by using hammer utlity with mirror-copy parameter. First we need to create two pfs, called for master and slave. I created them as /mymaster and /myslave. Master volume must be created with pfs-master and slave with pfs-slave parameter by using hammer utility. One thing to take in accounting here is that slave pfs must use master's shared-uuid number.
After creating mymaster and myslave pfs we have to use mirror-copy to start initial mirroring operation by using these pfs links , because mount_null can't access them without this copy operation.
After, running first mirror copy operation on our pfs links, we can mount our original hamme volumes where we want and associate them with the pfs links we created and sync (mirror-copy) our mounted hammer volumes.
You can use mount_null mount points to do your filesystems operations. You can see that slave volume (myslave) is read-only and you don't have write access to it. You can only use master volume (mymaster) to make your write operations.If you're looking an alternative filesystem to ffs on BSD systems, I suggest you to evaluate both ZFS on FreeBSD and Hammer on DragonFly BSD.