ZFS is a different file system than the old UFS. Best way to describe it is to think on it as a file system with LVM built on it. If you know LVM, you will understand what I am trying to say and recognize a lot of things here. If not, I promise that soon I will try to talk about it and compare ZFS, Linux LVM, and AIX LVM. For now, accept my simplistic explanation:
Logical Volume Manager, or LVM, is a way the separate the physical storage (think hard drives and networks drives and such things) from the storage seen by the user. In other words, what the users see is a pseudo disk (or logical volume if we are to use its terminology) of sorts which can be made of a collection of other disks, raid arrays, or just partitions of those disks. Now, while Linux LVM provides the logical disk which is then formatted and partitioned as they were normal disks, in ZFS these two steps are done become just one.
I think this will make sense using an example: in this Solaris 10 machine, we have two 73GB SCA hard drives. One of which, c0t0d0 (yes, Sun likes to call their drives differently than everybody else. We will talk about that in some other episode. For now, it suffices to say that d0 is the SCSI disk with SCSI ID 0), was formated as ZFS during the installation of the operating system and currently looks like this:
# df -h
Filesystem size used avail capacity Mounted on
boot/ROOT/root 67G 6.8G 40G 15% /
/devices 0K 0K 0K 0% /devices
ctfs 0K 0K 0K 0% /system/contract
proc 0K 0K 0K 0% /proc
mnttab 0K 0K 0K 0% /etc/mnttab
swap 1.9G 1.4M 1.9G 1% /etc/svc/volatile
objfs 0K 0K 0K 0% /system/object
sharefs 0K 0K 0K 0% /etc/dfs/sharetab
fd 0K 0K 0K 0% /dev/fd
boot/ROOT/root/var 67G 727M 40G 2% /var
swap 1.9G 32K 1.9G 1% /tmp
swap 1.9G 40K 1.9G 1% /var/run
boot 67G 176K 40G 1% /boot
boot/export 67G 20K 40G 1% /export
boot/export/home 67G 15G 40G 28% /export/home
#
Then we have c0t1d0, also a 73GB HD that we plan to add to the hard drive. For this machine, I would like to have the following layout:
/
/var
/tmp
/export/home (accounts)
/export/hosts (virtual hosts, in case we have them)
/export/images (as this may end up being a netboot/jumpstart server)
Each of those would reside in a separate partition. Some of that has already been taken care of by Solaris' default install, but we are left with /export/hosts and /export, which will be run off c0t1d0. Before ZFS, we would divide the hard drive using format into two partitions and would edit /etc/vfstab to mount the two partitions at boot time:
# cat /etc/vfstab
#device device mount FS fsck mount mount
#to mount to fsck point type pass at boot options
#
fd - /dev/fd fd - no -
/proc - /proc proc - no -
/dev/zvol/dsk/boot/swap - - swap - no -
/devices - /devices devfs - no -
sharefs - /etc/dfs/sharetab sharefs - no -
ctfs - /system/contract ctfs - no -
objfs - /system/object objfs - no -
swap - /tmp tmpfs - yes -
/dev/dsk/c0t1d0s4 /dev/rdsk/c0t1d0s4 /export/hosts ufs 2 yes -
/dev/dsk/c0t1d0s6 /dev/rdsk/c0t1d0s6 /export/images ufs 2 yes -
#
but, in ZFS things are a bit different. First we need to create the pseudo disk mentioned above. In ZFS terminology that is called a pool; I guess they want to remind us that you usually create it by adding disks to it. So we create the pool, which shall be named storagepool. By now you may have realized that boot is the name of the pool created during the installation of the operating system.
# zpool create storagepool c0t1d0
# df -h
Filesystem size used avail capacity Mounted on
boot/ROOT/root 67G 6.8G 40G 15% /
/devices 0K 0K 0K 0% /devices
ctfs 0K 0K 0K 0% /system/contract
proc 0K 0K 0K 0% /proc
mnttab 0K 0K 0K 0% /etc/mnttab
swap 1.9G 1.4M 1.9G 1% /etc/svc/volatile
objfs 0K 0K 0K 0% /system/object
sharefs 0K 0K 0K 0% /etc/dfs/sharetab
fd 0K 0K 0K 0% /dev/fd
boot/ROOT/root/var 67G 727M 40G 2% /var
swap 1.9G 32K 1.9G 1% /tmp
swap 1.9G 40K 1.9G 1% /var/run
boot 67G 176K 40G 1% /boot
boot/export 67G 20K 40G 1% /export
boot/export/home 67G 15G 40G 28% /export/home
storagepool 67G 19K 67G 1% /storagepool
#
You may have noticed that the pool we created, storagepool is also a filesystem: as soon as we created the pool, it becomes a filesystem mounted under /storagepool. That is different than what is done under Linux LVM, where you would first create the pool, then partition and format that pool into the partitions you are going to use, and then mount them using /etc/fstab (the Linux equivalent to /etc/vfstab and mount -a. In Solaris and ZFS, on the other hand, that is done with just one command. I mean, we did not even have to edit /etc/vfstab. The current list of ZFS mountpoints defined can be seen using zfs filelist:
# zfs list
NAME USED AVAIL REFER MOUNTPOINT
boot 26.5G 40.5G 176K /boot
boot/ROOT 7.46G 40.5G 18K legacy
boot/ROOT/root 7.46G 40.5G 6.75G /
boot/ROOT/root/var 727M 40.5G 727M /var
boot/dump 2.00G 40.5G 2.00G -
boot/export 15.0G 40.5G 20K /export
boot/export/home 15.0G 40.5G 15.0G /export/home
boot/swap 2G 42.5G 16K -
storagepool 89.5K 66.9G 1K /storagepool
#
If you remember, from storagepool we will be creating /export/hosts. That is done as follows:
# zfs create storagepool/hosts
# df -h
Filesystem size used avail capacity Mounted on
boot/ROOT/root 67G 6.8G 40G 15% /
/devices 0K 0K 0K 0% /devices
ctfs 0K 0K 0K 0% /system/contract
proc 0K 0K 0K 0% /proc
mnttab 0K 0K 0K 0% /etc/mnttab
swap 1.9G 1.4M 1.9G 1% /etc/svc/volatile
objfs 0K 0K 0K 0% /system/object
sharefs 0K 0K 0K 0% /etc/dfs/sharetab
fd 0K 0K 0K 0% /dev/fd
boot/ROOT/root/var 67G 727M 40G 2% /var
swap 1.9G 32K 1.9G 1% /tmp
swap 1.9G 40K 1.9G 1% /var/run
boot 67G 176K 40G 1% /boot
boot/export 67G 20K 40G 1% /export
boot/export/home 67G 15G 40G 28% /export/home
storagepool 67G 19K 67G 1% /storagepool
storagepool/hosts 67G 18K 67G 1% /storagepool/hosts
#
If we just leave at that, a mount point, /storagepool/hosts, is created. But, we really do not want that. We want to mount storagepool/hosts in /export/hosts, and mount we shall.
# zfs set mountpoint=/export/hosts storagepool/hosts
#
Now, it seems to be mounted where we want
# df -h
Filesystem size used avail capacity Mounted on
boot/ROOT/root 67G 6.8G 40G 15% /
/devices 0K 0K 0K 0% /devices
ctfs 0K 0K 0K 0% /system/contract
proc 0K 0K 0K 0% /proc
mnttab 0K 0K 0K 0% /etc/mnttab
swap 1.9G 1.4M 1.9G 1% /etc/svc/volatile
objfs 0K 0K 0K 0% /system/object
sharefs 0K 0K 0K 0% /etc/dfs/sharetab
fd 0K 0K 0K 0% /dev/fd
boot/ROOT/root/var 67G 727M 40G 2% /var
swap 1.9G 32K 1.9G 1% /tmp
swap 1.9G 40K 1.9G 1% /var/run
boot 67G 176K 40G 1% /boot
boot/export 67G 20K 40G 1% /export
boot/export/home 67G 15G 40G 28% /export/home
storagepool 67G 19K 67G 1% /storagepool
storagepool/hosts 67G 18K 67G 1% /export/hosts
#
Neat, huh? There is more we can do, like establish quotas and such stuff. But that will be for another episode.
Some parting thoughts
Remember that Sun call boot and storagepool pools. Even though we've only used one disk per pool, we could have used more. For instance, we could have grouped them together like in Linux LVM,
# zpool create anotherpool c0t0d0 c0t1d0
#
or, we could have made a pool where one of the disks mirrors the other, as in a raid 1
# zpool create anotherpool mirror c0t0d0 c0t1d0
#
Another option is instead of doing a mirror is to create a raid 5 (Sun claims it is a variation of raid 5, bigger, faster, better) using the raidz option and at least 3 drives (min for such raid),
# zpool create anotherpool raidz c0t0d0 c0t1d0 c0t2d0
#
We could also create hotspares for those arrays; check the man page for zpool for more info on that. I hope that will make you think on what ZFS can do and why I think it is rather neat.
