Something hammering MySQL (5M/s disk writes)?

[wbk]

Because of my problem with the size of Nextcloud’s oc_filecache, I have been paying more attention than usual to my server.

There is a constant write of (usually over) 5MB/s, that pauses when I stop MySQL.

The only recurring user that is not myself (as root) or MySQL itself, is ffsync. The processlist got ascending numbers only for ffsync’s processes:

MariaDB [(none)]> show processlist;
+-------+-------------+-----------------+--------+---------+------+--------------------------+------------------+----------+
| Id    | User        | Host            | db     | Command | Time | State                    | Info             | Progress |
+-------+-------------+-----------------+--------+---------+------+--------------------------+------------------+----------+
|     1 | system user |                 | NULL   | Daemon  | NULL | InnoDB purge coordinator | NULL             |    0.000 |
|     2 | system user |                 | NULL   | Daemon  | NULL | InnoDB shutdown handler  | NULL             |    0.000 |
|     3 | system user |                 | NULL   | Daemon  | NULL | InnoDB purge worker      | NULL             |    0.000 |
|     4 | system user |                 | NULL   | Daemon  | NULL | InnoDB purge worker      | NULL             |    0.000 |
|     5 | system user |                 | NULL   | Daemon  | NULL | InnoDB purge worker      | NULL             |    0.000 |
| 15461 | root        | localhost       | NULL   | Query   |    0 | Init                     | show processlist |    0.000 |
| 16166 | ffsync      | localhost:43260 | ffsync | Sleep   |  546 |                          | NULL             |    0.000 |
| 20019 | ffsync      | localhost:56798 | ffsync | Sleep   |  452 |                          | NULL             |    0.000 |
+-------+-------------+-----------------+--------+---------+------+--------------------------+------------------+----------+
8 rows in set (0.000 sec)

The writes are combined with continuous reading at about 1MB/s.

At first I ascribed it to trying to cleanup Nextcloud’s oc_filecache, which was down to 318M records, and then jumped back to the initial 321M records (I don’t know how MySQL rollbacks work, and if that was happening), but there is no related process in the list.

There are fewer than 10 Firefox instances syncing to this ffsync at any moment, and at this moment only four of those devices are turned on.

Anyone with an ffsync server running, to compare activity?

Or, even better, a way to measure disc reads/writes on a per-database basis?

[wbk]

In the meantime I have stopped nginx for a while, figuring that disk activity should drop when Firefox clients can not reach the service anymore.

Stopping nginx has no influence on disk writes, so it is nothing external that triggers the activity. I mentioned ffsync in the post title, but the mentions in the processlist must be red herrings, so I changed the title.

The server is quite busy as long as MySQL is running, and all but idle when I stop MySQL. Having a few Wordpress and some other apps served via nginx/MySQL seem to have no impact on idleness, nor do Matrix/Synapse + PostgreSQL .

Any suggestion on pinpointing the culprit?