If you are just running Ubunbtu with ZFS without poking into the details, you may not be aware of the scrubber running. For background information, and for the benefit of those who prefer to go their own way, this is all about that little scrubber.
A pool scrub operation is where the kernel runs through checking all of the data in a pool and makes any necessary repairs. Whilst ZFS does check the integrity of the data (using checksums) when performing reads, a regular scrub repairs these issues in advance.
It need only be run weekly for larger systems or monthly for normal systems (it’s a pretty arbitrary border line). And can be started manually with :-
# zpool scrub pool0
(And “pool0” being the name of the pool to scrub)
Whilst a scrub is going on in the background, the only effect on the system is that disk accesses to that pool will be slightly slower than normal. Usually not enough to notice unless you are benchmarking!
When in progress the output of zpool status pool0 will show the current state and how long it is expected to take to complete the scrub. Once finished the status will look like :-
# zpool status | grep scan:
scan: scrub repaired 0B in 0 days 09:19:27 with 0 errors on Sun Jun 21 10:36:28 2020
There are two aspects to ZFS that I will be covering here – checksums and error-correcting memory. The first is a feature of ZFS itself; the second is a feature of the hardware that you are running and some claim that it is required for ZFS.
By default ZFS keeps checksums of the blocks of data that it writes to later verify that the data block hasn’t been subject to silent corruption. If it detects corruption, it can use resilience (if any) to correct the corruption or it can indicate there’s a problem.
If you have only one disk and don’t ask to keep multiple copies of each block, then checksums will do little more than protect the most important metadata and tell you when things go wrong.
All that checksum calculation does make file operations slightly slower but frankly without benchmarks you are unlikely to notice. And it gives extra protection to your data.
For those who do not believe that silent data corruption exists, take a look at the relevant Wikipedia page. Everyone who has old enough files has come across occasional weird corruption in them, and whilst there are many possible causes, silent data corruption is certainly one of them.
Personally I feel like a probably unnoticeable loss of performance is more than balanced by greater data resilience.
I’m an enthusiast for ECC memory – my main workstation has a ton of it, and I’ve insisted on ECC memory for years. I’ve seen errors being corrected (although that was back when I was running an SGI Indigo2). Reliability is everything.
However there are those who will claim you cannot run ZFS without ECC memory. Or that ZFS without ECC is more dangerous than any other file system format without ECC.
Part of the problem is that those with the most experience of ZFS are salty old Unix veterans who would are justifiably contemptuous of server hardware that lacks ECC memory (that includes me). We would no sooner consider running a serious file server on hardware that lacks ECC memory than rely on disk ‘reliability’ and not mirror or RAID those fallible pieces of spinning rust.
ZFS will run fine without ECC memory.
But will it make it worse?
It’s exceptionally unlikely – there are arguable examples of exceptionally esoteric failure conditions that may make things worse (the “scrub of death”) but I side with those who feel that such situations are not likely to occur in the real world.
And as always, why isn’t your data backed up anyway?
You will be asked for the passphrase as it is created. Forgetting this is extremely inadvisable!
One created, reboot to check that :-
You get prompted for the passphrase (as of Ubuntu 20.04 you do).
That the encrypted filesystem gets mounted automatically (likewise).
At this point you should be able to create the filesystems for the relevant home directories :-
zfs create rpool/USERDATA/ehome/root
rsync -arv . /ehome/root
zfs set mountpoint=/root rpool/USERDATA/ehome/root
(An error will result as there is something already there but it does the important bit)
zfs set mountpoint=none rpool/USERDATA/root_xyzzy
(A similar error)
Repeat this for each user on the system, and reboot. See if you can login and your files are present.
This leaves the old unencrypted home directories around (which can be removed with zfs destroy -r rpool/USERDATA/root_xyzzy). It is possible that this re-arrangement of how home directories work will break some of Ubuntu’s features – such as scheduled snapshots of home directories (which is why the destroy command needs the “-r” flag before).
A number of those who have experimented with Ubuntu’s ZFS install option (which as of 20.04 is marked as “experimental”) have expressed bewilderment over the number of filesystems created :-
The short answer as to why is that there are two schools of thought amongst grizzled old Unix veterans as to whether one big filesystem should be the way to go or lots of little ones. There are pros and cons to both approaches, and whilst I have a preference for lots of filesystems (especially on servers), I don’t care enough to change it on a laptop install.
Even though those who insist on one big filesystem are wrong.
As to the longer explanation …
A long time ago – the 1970s or the 1980s – Unix systems lacked sophisticated disk management software, and the disks were very much smaller (I started off with 80Mbyte disks and no that isn’t a typo, and many started with much smaller disks). On larger Unix servers, you couldn’t fit everything onto one disk, so we got used to splitting up the filesystem into many separate filesystem – / on one disk partition (or slice), /usr on another, /var on a third, /home on yet another, etc.
These very frequently got further subdivided – /var/mail, /var/tmp, /var/spool, etc. as Unix servers got larger and busier.
Those days are long past, and nobody is keen to go back to those days so why do some still like to split things up?
The Fringe Benefits of Splitting
It turns out that there was a fringe benefit to splitting up the filesystems – disk space exhaustion on one wouldn’t cause a problem elsewhere. For example if a mail server had a separate /var/spool/mail filesystem for operating within it would still continue to operate if /var filled up; similarly a DNS server wouldn’t crash and burn if it had a /var/named filesystem and /var filled up.
Both of those examples are known to me personally – and there are many other examples.
Of course there is also a downside – if you create a separate /var/spool/mail filesystem you need to make sure it is large enough to operate not just normally but in reasonable exceptional circumstances. Or your mail server crashes and burns.
On the other hand, if you don’t separate things out then when something goes berserk and fills up all the disk space then you will have a good deal of trouble actually logging in to fix things.
In a sense, the “everything in one” camp and “lots of little filesystems” camp are determined by what troubles we’ve seen over the years (and in some cases decades).
With something like ZFS you can set quotas to limit the size of any filesystem so managing the sizes of these separate filesystems is a great deal easier than it ever was in the past! Ubuntu does not set quotas by default on a desktop installation; for a server it may well be worth checking quotas and setting them appropriately.
And Snapshots …
One of the other things that Ubuntu does with ZFS and filesystem snapshots (we’ll worry about what those are another time) is to offer to rollback a broken update. People worry that upgrading their system will break things and the ability to quickly revert to the previous state is very comforting.
But the Unix file layout “standard” and the later Linux file layout standard were not designed with snapshots in mind, and simply rolling back the whole of “/” would have negative effects – not least you would lose any file changes you had made in /home and any mail stashed away in /var/mail.
So to implement the ability to rollback updates requires numerous separate filesystems to avoid losing important data.
It is also likely that it would be beneficial to tune separate filesystems for different requirements.
In short, don’t worry about it. It’ll have very little effect on your operation of a normal Ubuntu machine unless you choose to take advantage of it. And it makes possible certain features that you will probably like – such as the ability to revert updates.
On occasions, I have run into issues where mounting a filesystem from /etc/fstab fails on a reboot because it depends on something else happening first. The easiest example to recall is when mounting a conventional filesystem constructed from a ZPool block device – the block device isn’t ready until ZFS has finished starting which often occurs after the filesystem mounts are attempted.
The fix is dead simple; just add the option “_netdev” to the options field in /etc/fstab and the problem is sorted :-