Drop background readahead on boot

Starting Tails 3.9 from DVD is twice slower than 3.8 (3:55 vs. 2:12 on my hardware), presumably because my “common sense” (sic) did not make me filter out enough stuff from the SquashFS sort file. I did not measure things nor look at the bootchart but it seems as if the X.Org startup was blocked for 2 minutes, waiting for the background readahead to complete, which is kinda ironic.

I think we should:

1. keep the SquashFS sort file for now, hoping it is still useful
2. try removing the part about BG_FILES from config/chroot_local-includes/lib/live/config/0000-readahead and measure boot time from DVD and USB, both in a problematic case (3.9) and optimal case (like the squashfs.sort used for 3.8)
3. if removing background readahead helps, try removing the foreground readahead as well and measure things again

Feature Branch: feature/15915-remove-readahead

Attachments

• dmesg.txt
• tails-amd64-feature_15915-remove-readahead-3.12-20190106T1143Z-79907a506c.buildlog

Related issues

• Blocked by #16134 (closed)

Original created by @intrigeri on 15915 (Redmine)