Hi all!

New here, signed up just new, to discuss an interesting mariadb behaviour we are seeing, related to mariadb unexpectedly using swap space.

Look at this example:

RHEL 8.10, running mariadb-server-utils.x86_64 3:10.5.22-1.module+el8.8.0+20134+a92c7654:

[user@db ~]$ free -g
              total        used        free      shared buff/cache   available
Mem:            187           9          11           0         165         175
Swap:             3           3           0

and:

top - 08:52:46 up 39 days, 15:50, 2 users, load average: 1.70, 2.05, 1.99
Tasks: 616 total,   2 running, 614 sleeping,   0 stopped,   0 zombie
%Cpu(s): 6.0 us, 0.8 sy, 0.0 ni, 88.2 id, 4.7 wa, 0.1 hi, 0.1 si, 0.0 st
MiB Mem : 191529.7 total, 11761.7 free,   9764.9 used, 170003.1 buff/cache
MiB Swap:   4096.0 total,   1006.1 free,   3089.9 used. 179429.5 avail Mem

    PID USER      PR NI    VIRT    RES    SHR S %CPU %MEM     TIME+ COMMAND       SWAP
1952811 mysql     20   0   14.4g   5.0g 15172 S 65.7   2.7   9663:14 mysqld        1.7g
2934963 root      20   0    5208   2056   1408 S   8.9   0.0 10:39.29 gzip          0
2934962 root      20   0   33612   8268   6960 S   7.9   0.0   2:35.31 mysqldump     0

while:

[user@db ~]$ cat /proc/sys/vm/swappiness
1

We could (we should..) allocate more ram to mariadb, but the point is: there is plenty of RAM available, and there always has been, since boot. And yet, mariadb is using all swap.

I guess this has no performance impact, probably the swapped pages are not actually used, but zabbix does not like 100% swap usage. And frankly: neither do I.

Why? Can anyone here explain? Swappiness set to 1...is mariadb ignoring swappiness..? Is there something else we can configure..?

I read some older posts on ram/swap, and here is some data, requested in that older thread:

[user@db ~]$ sudo ps -o pid,vsz,comm `pgrep mysqld`
PID VSZ COMMAND
1952811 15117612 mysqld

and

[user@db ~]$ for file in /proc/*/status ; do awk '/VmSwap|Name/{printf $2 " " $3}END{
> print ""}' $file; done | sort -k 2 -n -r | grep mysql
mysqld 1982672 kB

We are seeing similar behaviour on another machines, running more recent mariadb version: 10.5.22-MariaDB MariaDB Server:

[user@db_another ~]$ free -g
              total        used        free      shared buff/cache   available
Mem:             62          40           0           1          21          20
Swap:             4           3           1

Again: enough RAM available, and swap is used 3/4, mostly by MariaDB.

Hope the provided information is enough. Let me know if there is anything else I can provide. Looking forward for any insight or help on the matter. :-)

Thanks!
MJ