Ubuntu ZFS: Checksums and ECC

 Computing, Linux  No Responses »
May 172020
 

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.

Checksums

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.

Error-Correcting Memory

(Henceforth “ECC”)

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.

Not really.

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?

Ubuntu ZFS: Encryption

 Computing, Linux  No Responses »
Apr 262020
 

Experimenting with Ubuntu’s “new” (relatively so) ZFS installation option is all very well, but encryption is not optional for a laptop that is taken around the place.

Whether I should have spent more time poking around the installer to find the option is a possibility, but post-install enabling encryption isn’t so difficult.

The first step is to create an encrypted filesystem – encryption only works on newly created filesystems and cannot be turned on later :-

zfs create -o encryption=on \
  -o keyformat=passphrase \
  rpool/USERDATA/ehome

You will be asked for the passphrase as it is created. Forgetting this is extremely inadvisable!

One created, reboot to check that :-

  1. You get prompted for the passphrase (as of Ubuntu 20.04 you do).
  2. 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
cd /root
rsync -arv . /ehome/root
cd /
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).

But it’s getting there.

Ubuntu ZFS: Why All The Filesystems?

 Computing, Linux  No Responses »
Apr 262020
 

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 …

Some History

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.

Finally

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.

Messing With Rofi: Inserting Unicode

 Computing, Linux, Working Notes  No Responses »
Feb 292020
 

I used to be able to remember all of the keyboard short-cuts I’d set up to insert ‘fun’ (or useful) Unicode characters … but my memory isn’t quite what it used to be, and I happened to catch a video that mentioned a menu to insert Unicode characters.

So I set out to create my own …

#!/bin/zsh
#
# Run a menu of Unicode characters to put into the clipboard

read -r -d '' menu << END
Þ       Thorn (Sym-t)
Þ	Capital thorn (Sym-T)
✓	Tick (Sym-y)
✔       Alternate tick (Sym-Y)
π	pi (Sym-p)
Π       PI (Sym-P)
★       Star
🖕      Finger
END

selectedchar=$(echo $menu |\
  rofi -dmenu -l 20 -fn misc-24 -p "Unicode inserter" |\
  awk '{print $1}')
if [ -z "$selectedchar" ]
then
  notify-send "Unicode Inserter" "Cancelled"
else
  printf "%s" $selectedchar | xclip -selection clipboard
  notify-send "Unicode Inserter" \
    "Character $selectedchar now in clipboard"
fi

This script :-

  1. Creates a variable with a whole pile of text inside it. The original script contains a lot longer list, but including it would be a) boring and b) give too much away. As it stands, the format of each line is pretty much anything you want as long as the first character is the Unicode character followed by whitespace.
  2. Runs rofi with the variable as input and selects the first field of the response.
  3. Guards against the empty selection (when the menu was cancelled) for neatness mostly.
  4. Prints the selected character (without a newline) into xclip so it can be pasted in. I did try using xdotool to type it directly into the active window, but this didn’t always work so well (i.e. xdotool couldn’t “type” some of the more esoteric characters).
  5. And uses notify-send to alert the dumb user (me!) that something has happened.

Lastly, to make this useful, I added an entry to my sxhkd configuration :-

super + F11
  /site/scripts/m-unicode-insert

Virtual Cores Are Not Physical Cores

 Computing  No Responses »
Feb 242020
 

Every so often, I tune into a video on some form of virtualisation which perpetuates the myth that ‘virtual cores’ that you allocate to a virtual machine are equivalent to the physical cores that the host has. In other words if you create a virtual machine with two cores, that is two cores that the rest of the host cannot use.

Preposterious.

Conceptually at least, a core is a queue runner that takes a task on a queue, runs that task for a while, and then sticks that task back on the queue. Except for specialised workloads, those cores are very often (even mostly) idle.

To the host machine, tasks scheduled to run on a virtual core are just tasks to be performed waiting in the queue; ignoring practicality, there is no reason why there should not be more virtual cores in a virtual machine than there are in the host machine.

If you take a look at the configuration of my virtual Windows machine in VirtualBox :-

You see :-

  1. I’ve allocated 8 virtual cores to this machine. I rarely use this machine (although it is usually running), but it does not take much resources to run idle cores.
  2. VirtualBox arbitrarily limits the number of cores I can allocate to the virtual machine to the number of threads my processor has; it also has a warning at the number of cores my processor has but doesn’t stop me allocating virtual cores in the “red” zone.

Qemu on the other hand has no such qualms about launching a virtual machine with 64 cores – well in excess of what my physical processor has.

Of course you have to be sensible, but creating a virtual machine with 4 cores does not make four cores unavailable to your host machine. If a virtual machine is idle, it won’t be running much (no machine is ever completely idle) on your real cores.

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