blockdev: Use --direct-io=on for losetup

[cgwalters]

This is what other projects do; it avoids double buffering and is generally a good idea.

[jeckersb]

+1 for being explicit here. One thing to note is that I'm pretty sure this is not strictly necessary per util-linux/util-linux@5b3bb7d -- the manpage was wrong and 'on' was always the default. The manpage on stream9 still has the older incorrect version.

[cgwalters]

the manpage was wrong and 'on' was always the default.

The manpage was saying --direct-io is the same thing as --direct-io=on AFAICS - not that it's on by default.

Testing this

# bootc status --json | jq -r .status.booted.image
{
  "image": {
    "image": "quay.io/centos-bootc/fedora-bootc:eln-1709537055",
    "transport": "registry"
  },
  "version": "39.20240304.0",
  "timestamp": null,
  "imageDigest": "sha256:610131941967d62b3819756b6246a415f9dfff521bd229b5c75299393f51babc"
}
# rpm -q util-linux
util-linux-2.40-0.11.rc1.eln136.x86_64
# truncate -s 10G /var/tmp/foo
losetup -f -P /var/tmp/foo 
# losetup 
NAME       SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE                                                                                                            DIO LOG-SEC
/dev/loop1         0      0         0  0 /var/tmp/foo                                                                                                           0     512
/dev/loop0         0      0         1  1 /sysroot/ostree/deploy/default/deploy/e1455b056e30507ac9f51cc1c195ea1b29c9203a62e2f1309b9b0b84cd33f355.0/.ostree.cfs   1    4096
#

Notice the composefs mount is using DIO but the loopback mount I did is not.

[jeckersb]

Ah gotcha. My English parser is buggy 😆

[mvo5]

Yeah, the code in https://github.com/util-linux/util-linux/blob/master/sys-utils/losetup.c#L687 also indicates the default is "off". But this is super nice, we will try this for osbuild too.