I have txindex=0.
I propose that's wallet.dat I/O because I see portions of increasing in "iotop -to -a" output.
And each portion is about 480 Mb. Exactly as wallet.dat size. That's why.
To me it looks like somebody is repeatedly calling "backupwallet" on your server.
Anyway, here's the quickest way to find out:
1) create DB_CONFIG file with one line "set_lg_dir database"
2) start the bitcoind with -privdb=0
3) use db_stat to monitor it live
4) download (and read or at least skim) the documentation from docs.oracle.com
The easiest way to fix the large wallet.dat problems is by adding two more physical disk drives to your server: one for wallet.dat one for its log directory ("database" by default, but it is best to change it). Adding SSD drives doesn't help much because it uses
https://en.wikipedia.org/wiki/Write-ahead_logging and this is exactly pessimal application for flash memory: all writes and no reads.