DragonFly BSD

how to implement hammer pseudo file system( pfs ) slave mirroring from pfs master

Scenario

I have two 500 GB hard disks both with the Hammer file system. I want to create a master PFS in one hard disk and a slave PFS in the other disk. I want to mirror the data continuously from the master PFS to the slave PFS. This will help me avoid long 'fsck' and RAID parity rewrite times after an unclean shut down, and also will give me a setup somewhat like RAID 1.

Creating the master PFS on Disk 1

The Hammer file systems on Disk 1 and Disk 2 are mounted in '/etc/fstab' according to the following.

/dev/ad4s1h             /Backup1        hammer  rw              2       2
/dev/ad6s1h             /Backup2        hammer  rw              2       2

Go to the Hammer file system on Disk 1. We will be creaing a master PFS called 'test' and will be mounting it using a null mount. If you don't have a directory called 'pfs' under the Hammer file system you should create it.

# pwd
/Backup1
# mkdir pfs

If you already have the pfs directory under the Hammer file system you can skip the above step and continue.

# hammer pfs-master /Backup1/pfs/test
Creating PFS #3 succeeded!
/Backup1/pfs/test
sync-beg-tid=0x0000000000000001
sync-end-tid=0x000000013f644ce0
shared-uuid=9043570e-b3d9-11de-9bef-011617202aa6
unique-uuid=9043574c-b3d9-11de-9bef-011617202aa6
label=""
prune-min=00:00:00
operating as a MASTER
snapshots dir for master defaults to <fs>/snapshots

Now the master PFS 'test' is created. Make a note of its 'shared-uuid' because we will need to use that to create the slave PFS for mirroring. You can mount the PFS under the Hammer file system on Disk 1 by doing the following.

# mkdir /Backup1/test

Now Edit '/etc/fstab' to contain the following line.

/Backup1/pfs/test      /Backup1/test    null    rw              0       0

Now mount the PFS by doing.

# mount -a
# mount |grep test
/Backup1/pfs/@@-1:00003 on /Backup1/test (null, local)

Creating the slave PFS on Disk 2.

Note that we must use the 'shared-uuid' of the master PFS to enable mirroring.

# hammer pfs-slave /Backup2/pfs/test shared-uuid=9043570e-b3d9-11de-9bef-011617202aa6
Creating PFS #3 succeeded!
/Backup2/pfs/test
sync-beg-tid=0x0000000000000001
sync-end-tid=0x0000000000000001
shared-uuid=9043570e-b3d9-11de-9bef-011617202aa6
unique-uuid=97d77f53-b3da-11de-9bef-011617202aa6
slave
label=""
prune-min=00:00:00
operating as a SLAVE
snapshots directory not set for slave

The slave PFS is not mounted but a symlink can be created in the root Hammer file system to point to it.

# ln -s /Backup2/pfs/test /Backup2/test
# ls -l /Backup2/test
lrwxr-xr-x  1 root  wheel  17 Oct  8 12:07 /Backup2/test -> /Backup2/pfs/test

(This step is optional, the PFS can be read through the original magic symlink /Backup2/pfs/test.)

Copying contents from PFS on Disk 1 to PFS on Disk 2 to enable mirroring.

The slave PFS will be accessible only after the first 'mirror-copy' operation.

# touch /Backup1/test/test-file
# ls /Backup1/test/
test-file
# sync

We do the "sync" so that the file creation operation in flushed from the kernel memory. Mirroring works only on operations flushed from the kernel memory. The slave PFS will be accessible only after the first mirroring operation.

# hammer mirror-copy /Backup1/test /Backup2/pfs/test
histogram range 000000013f6425fd - 000000013f644d60
Mirror-read: Mirror from 0000000000000002 to 000000013f644d60
Mirror-read /Backup1/test succeeded

# ls /Backup2/test/
test-file

Enabling continuous mirroring.

The hammer mirror-stream will automatically restart if the connection is lost so you only need to start it up once at boot.You can do this with an @reboot entry in the crontab.

@reboot hammer mirror-stream /Backup1/test /Backup2/test