====== Reduce SD Card wearing ======
[[https://www.debian-administration.org/article/661/A_transient_/var/log|Transient log]] \\
[[https://wiki.gentoo.org/wiki/SDCard|Finding SD card erase size]] \\

===== Check who is writing =====
Start logging disc access and kernel log:
<code bash>
sysctl vm.block_dump=1
dmesg -e -T -w 
</code>

Stop logging:
<code bash>
sysctl vm.block_dump=0
</code>


===== Set noop elevator =====
<file | /etc/rc.local>
...
echo noop > /sys/block/mmcblk0/queue/scheduler
exit 0
</file>

===== Use tmpfs =====
<file | /etc/fstab>
tmpfs   /tmp                     tmpfs   defaults,nosuid,nodev,mode=1777,size=10%        0       0
tmpfs   /var/tmp                 tmpfs   defaults,nosuid,nodev,mode=1777,size=10%        0       0
tmpfs   /var/cache/apt/archives  tmpfs   defaults,nosuid,nodev,mode=1777,size=80%        0       0
</file>

Move some volatile dirs to tmp folder:
<code bash>
rm -rf /var/run /var/lock
ln -s /run /var/run
ln -s /run/lock /var/lock
</code>

<code bash>
rm -rf /var/lib/dhcp/ /var/spool
ln -s /tmp /var/lib/dhcp
ln -s /tmp /var/spool;
</code>


===== Disable logging to disc =====

<code bash>
systemctl stop mosquitto
systemctl disable mosquitto
</code>

<code>apt-get remove anacron</code>

Remove syslog
<code>dpkg --purge rsyslog</code>

Use syslog from busybox package with circular log buffer in RAM:
<code>
apt-get install busybox-syslogd
</code>

<file /etc/default/busybox-syslogd>
SYSLOG_OPTS="-C256 -L -R remote.host.org"
KLOG_OPTS=""
</file>

Use journald and configure it to use RAM:
<file | /etc/systemd/journald.conf>
Storage=volatile
Compress=yes
</file>


===== Run fstrim weekly =====
Check TRIM support
<code>sudo hdparm -I /dev/sda | grep "TRIM supported"</code>

<file | /etc/systemd/system/fstrim.service>
[Unit]
Description=Discard unused blocks

[Service]
Type=oneshot
ExecStart=/sbin/fstrim -av
</file>

<file | /etc/systemd/system/fstrim.timer>
[Unit]
Description=Discard unused blocks once a week
Documentation=man:fstrim

[Timer]
OnCalendar=weekly
AccuracySec=1h
Persistent=true

[Install]
WantedBy=multi-user.target
</file>
<code bash>
systemctl daemon-reload
systemctl enable fstrim.timer
</code>

===== Broken SD Card =====
Bad magic number in super-block

<code>
[ 7000.203888] F2FS-fs (sda2): Magic Mismatch, valid(0xf2f52010) - read(0x0)
[ 7000.203926] F2FS-fs (sda2): Can't find valid F2FS filesystem in 1th superblock
</code>

<code>
e2fsck /dev/sda2
e2fsck 1.42.12 (29-Aug-2014)
ext2fs_open2: Bad magic number in super-block
e2fsck: Superblock invalid, trying backup blocks...
/dev/sda2: recovering journal
e2fsck: unable to set superblock flags on /dev/sda2


/dev/sda2: ***** FILE SYSTEM WAS MODIFIED *****

/dev/sda2: ********** WARNING: Filesystem still has errors **********
</code>

To get superblock backups, simulate fs creation
<code>sudo mke2fs -n /dev/xxx</code>
Then run fsck with superblock backup
<code>sudo e2fsck -b block_number /dev/xxx</code>