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How do I completely negate disk writes? (self.raspberrypi)

submitted 1 year ago* (last edited 1 year ago) by GustavoM to c/[email protected]

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Title. Asides from setting the whole disk to read-only. (It's not an option since apparently docker dislikes that for some reason.). I'm trying to increase my microsd's lifespan.

For instance, here's my current /etc/fstab

tmpfs /tmp tmpfs rw,size=10M,nr_inodes=5k,noexec,nodev,nosuid,uid=gus,gid=notgus,noatime,mode=1700

tmpfs /var/log tmpfs rw,size=10M,nr_inodes=5k,noexec,nodev,nosuid,uid=gus,gid=notgus,noatime,mode=1700

tmpfs /var/tmp tmpfs rw,size=0M,nr_inodes=5k,noexec,nodev,nosuid,uid=gus,gid=notgus,noatime,mode=1700

tmpfs /var/run tmpfs rw,size=0M,nr_inodes=5k,noexec,nodev,nosuid,uid=gus,gid=notgus,noatime,mode=1700

tmpfs /var/spool tmpfs rw,size=0M,nr_inodes=5k,noexec,nodev,nosuid,uid=gus,gid=notgus,noatime,mode=1700

tmpfs /var/lock tmpfs rw,size=0M,nr_inodes=5k,noexec,nodev,nosuid,uid=gus,gid=notgus,noatime,mode=1700

tmpfs /var/cache tmpfs rw,size=0M,nr_inodes=5k,noexec,nodev,nosuid,uid=gus,gid=notgus,noatime,mode=1700

Thanks in advance.

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[–] theit8514 6 points 1 year ago (1 children)

This is what overlayfs is designed for. You can turn it on in the raspi-config tool under performance. While the overlayfs is on, all writes will just happen in memory. You should also have an update-motd script which will let you know when overlayfs is on.

$ cat /etc/update-motd.d/80-overlay
#!/bin/sh

str=$(mount | grep ' on / ')
if echo $str | grep -q 'overlay'; then
        printf "\n\e[5m\e[1;31m------ INFO: / MOUNTED WITH OVERLAY ------\e[0m\n\n"
elif echo $str | grep -q 'rw'; then
        printf "\n\e[1;32m++++++ INFO: / MOUNTED READ-WRITE ++++++\e[0m\n\n"
else
        printf "\n\e[5m\e[1;31m!!!!!! WARNING: / UNKNOWN STATE !!!!!!\e[0m\n\n"
fi

[–] GustavoM 1 points 1 year ago* (last edited 1 year ago) (1 children)

What if my board has barely any available ram for that? (Right now its running sway, docker w/ nextdns and playing a 24/7 livestream through ffplay/yt-dlp and the available ram varies between 90MiB to 300 MiB.)

[–] theit8514 2 points 1 year ago

It would work like the existing tmpfs that you already have. It overlays the tmpfs over the readonly filesystem mount:

/dev/mmcblk0p2 on /media/root-ro type ext4 (ro,relatime)
tmpfs-root on /media/root-rw type tmpfs (rw,relatime)
overlayroot on / type overlay (rw,relatime,lowerdir=/media/root-ro,upperdir=/media/root-rw/overlay,workdir=/media/root-rw/overlay-workdir/_)

df will tell you how much ram has been reserved for the tmpfs and how much has been "written." On my 4GB Pi that is ~1.5GB. Mine is usually pretty idle and has been up for 40 days, so at 12% used ~1.5GB will last me quite a bit.

$ df /media/root-*
Filesystem     1K-blocks    Used Available Use% Mounted on
/dev/mmcblk0p2  59839564 1857276  55517464   4% /media/root-ro
tmpfs-root       1964220  234828   1729392  12% /media/root-rw