Maintenance and optimisation¶

Practical 5.8 maintenance-and-optimisation-annex.pdf · PDF

Has your phone ever said storage almost full? And have you ever installed a "cleaner" or "booster" app to fix it? Keep that app in mind. By the end of this page you will know what it really did — and why it is one of the most dangerous things you can install.

A machine that works every day gets dirty without anybody doing anything wrong. In Configuring the system by command you set deb-lab up. Now you keep it clean and fast, with the system's own tools — and you learn to judge a tool before you hand it the keys.

What maintenance is¶

Back in the kitchen. A kitchen that cooks every day collects empty boxes in the pantry, old order tickets on the spike, and tools nobody uses on the countertop.

Maintenance is the regular tidy-up that keeps a system working well: nothing new is cooked, but tomorrow's service is faster. Optimisation goes one step further: arrange things so the system does less work for the same result.

A system collects dirt in three places:

Who you let in: the CCleaner story¶

CCleaner was, for years, the most famous cleaner for Windows. In July 2017 the antivirus company Avast bought its maker, Piriform. On 15 August 2017, version 5.33 was published on the official site, signed with the official certificate. It had a backdoor inside: attackers had broken into Piriform's build machines. About 2.27 million people installed it before it was found a month later.

They had all done the "right" thing: official site, signed installer. Why was a cleaner such a good target? Because it runs as administrator, on millions of machines, and nobody watches what a cleaner deletes.

A maintenance tool is a stranger you give the keys of the whole kitchen to. Before you install one, ask five questions:

1. What does it do that the system cannot already do?
2. Who makes it, and where does it come from?
3. What rights does it ask for?
4. Can I undo what it does?
5. Is it still maintained?

Cleaning the disk: measure, find, clean, measure¶

The golden rule: measure before you touch anything. Otherwise you will never know what the clean-up achieved.

flowchart LR
    M1["<b>1 · Before</b><br>df -h /"] --> F["<b>2 · Find</b><br>sudo ncdu /"]
    F --> C["<b>3 · Clean</b><br>apt clean · apt autoremove<br>journalctl --vacuum-time"]
    C --> M2["<b>4 · After</b><br>df -h /<br>the difference is the result"]

1 · Before. On the 25 GB disk of deb-lab, typically 6–9 GB are used. Write down Size, Used, Avail and Use%.

df -h /

2 · Find the space. du (disk usage) tells you how big a directory is. ncdu is du with a menu: you walk the tree with the arrows, Enter goes into a directory, q quits.

sudo apt install ncdu
sudo ncdu /          # without sudo, many directories cannot be read

/usr is always the biggest: that is the system itself. /var is where the dirt is: caches and logs.

3 · Clean, with Debian's own tools:

sudo du -sh /var/cache/apt/archives   # the .deb files APT downloaded
sudo apt clean                        # deletes them: they are already installed
sudo apt autoremove                   # removes packages nothing needs any more: READ THE LIST
sudo journalctl --disk-usage          # e.g. Archived and active journals take up 8M in the file system.
sudo journalctl --vacuum-time=2weeks  # keep only the last two weeks of logs

None of the three touches your own files. autoremove is the only one that removes software, so it is the one to read.

4 · After. df -h / again. On a young install the gain is often small — tens or hundreds of MB.

The start-up: a stopwatch on the relay race¶

Remember the boot relay race from Unit 1: firmware → bootloader → kernel → services. systemd-analyze puts a stopwatch on it.

systemd-analyze                  # Startup finished in ...s (kernel) + ...s (userspace) = ...s
systemd-analyze blame | head -5  # units, slowest first
systemd-analyze critical-chain   # the chain that decided the total time

With UEFI, the first line also shows (firmware) and (loader): the first two runners of the relay. A real sample of blame:

$ systemd-analyze blame | head -3
73ms systemd-localed.service
34ms systemd-hostnamed.service
33ms systemd-logind.service

To see everything that starts at boot — services you met in Services and systemd:

systemctl list-unit-files --type=service --state=enabled
systemctl status some-name       # what is this one, and does deb-lab need it?

Investigate before you disable. A service you do not recognise may be the one that brings up the network.

Defragmentation and TRIM¶

The hard disk: files in pieces¶

Picture a strip of 12 boxes on a hard disk. File A takes boxes 1–4, file B boxes 5–8. You delete A. Now you write C, six boxes long: it goes into 1–4 and 9–10.

flowchart LR
    subgraph HDD ["Hard disk after deleting A and writing C"]
        direction LR
        C1["C<br>1–4"] --- B["B<br>5–8"] --- C2["C<br>9–10"] --- F["free<br>11–12"]
    end

A file is fragmented when its pieces are stored in places that are not next to each other.

On a hard disk, the head has to travel between the pieces — a seek of about 10 ms each (Unit 1). Defragmentation moves the pieces together again. Linux rarely needs it: the ext4 file system avoids most fragmentation.

Does an SSD care where the pieces are? No: there is no head, no travel. Defragmenting it only adds writes, and flash cells wear with writes.

The SSD: which blocks are free?¶

An SSD has a different problem. When you delete a file, the file system only crosses its name off a list. The drive still believes those blocks hold data — and flash must be erased before it is written again.

TRIM is the message from the operating system to an SSD saying "these blocks are free": the drive can erase them in advance, so later writes are fast.

Check it on deb-lab:

lsblk -D                       # zeros in DISC-GRAN and DISC-MAX: the disk does not accept discard
sudo fstrim -v /
systemctl status fstrim.timer  # "Discard unused filesystem blocks once a week"

On a virtual disk created without the discard option, fstrim answers:

fstrim: /: the discard operation is not supported

That is normal. Your VM's disk is a file on the host, so the host is the one that should take care of the real SSD. On a real SSD, fstrim -v / prints how many bytes were trimmed.

The same jobs on Windows¶

The two repair commands, in the order Microsoft gives, in a terminal opened as administrator:

DISM.exe /Online /Cleanup-image /Restorehealth
sfc /scannow

sfc (System File Checker) compares the protected system files with a known good copy and replaces the damaged ones. DISM first repairs the copy that sfc uses. sfc /scannow takes minutes; the usual final message is Windows Resource Protection did not find any integrity violations.

Judging two tools¶

In Part C of the Exercise sheet (Practical 5.8) you apply the five questions to two utilities from Debian's own repository: ncdu, and BleachBit, a graphical cleaner (sudo apt install bleachbit). Open BleachBit as a normal user, tick two or three items and press Preview — never Clean.

A verdict without a reason is worth nothing. "Remove it, the system already does it" is a complete answer. Then act on it: sudo apt purge name and sudo apt autoremove.

Key takeaways¶

• Maintenance keeps a system working well; optimisation makes it do less work. Dirt piles up on the disk, in the start-up and in the drive itself.
• Always measure before and after (df -h /), find the space with du or ncdu, and clean with the system's own tools: apt clean, apt autoremove (read the list), journalctl --vacuum-time.
• systemd-analyze times the boot; blame lists durations, critical-chain shows what really decided the total.
• An HDD is defragmented because its head travels; an SSD gets TRIM (fstrim, weekly by fstrim.timer) and is never defragmented.
• On Windows: Storage Sense, Startup apps, Optimize Drives, and DISM then sfc /scannow to repair system files.
• A maintenance tool runs with administrator rights: ask the five questions before installing it. CCleaner 5.33 came from the official site.

Quick check¶

Next: fill box H of your cheat sheet — What a development machine needs.