How do you scale dovecot for good performance with Roundcube webmailer in front? (hitting limits without exhausting resources)

[Malte Schmidt]

Good day,

I am currently setting up/debugging a webmailer-only setup using Roundcube (latest version) with Dovecot (2.3.20, latest as of now).

Generally speaking the setup performs OK, I wrote a JMeter Roundcube script to test the Roundcube. This includes actions like listing mails and fetching them. I am hitting a somehow dubious limit of 600 req/s though. (This includes all kinds of HTTP calls to Roundcube, not only the ones triggering IMAP). When the dovecot becomes unavailable, the performance without mails jumps to 7700 req/s. Testing is done with 4 JMeter-servers and one client. 

My setup is a Dovecot with MySQL (Percona XtraDB) backend. Mails are encrypted with mail_crypt and EC keys. Authentication is done in parallel (auth_cache_verify_with_workers=yes). Where possible min_available_processes have been set equal to the threads available on the Dovecot machine (64).

Hardware is a 64 thread Xeon CPU at 2.10 GHz, 96 GB RAM, SSDs as backing storage IOPS read 40000/write 13000.

What settings do you recommend and how was your experience with Roundcube and its performance in general (what should be possible with that kind of HW?)?

Best regards and thanks in advance!

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