1.     apx 10 days back upgraded both mx1 and mx2 to dovecot 2.3.1_2 from 2.3.0   (service dovecot stop, portmaster upgrade, service dovecot start)
2.     both systems ran ok with no errors for 10 days.
3.     Last night I shutdown mx2 and restarted it a few hours later
4.     within minutes i was getting the following types of errors on mx2

Hi,

[Formatting is a bit rough, replying from a trimmed digest email]

> Message: 1
> Date: Fri, 6 Apr 2018 15:04:35 +0200
> From: Michael Grimm <trashcan at ellael.org>
> To: Dovecot Mailing List <dovecot at dovecot.org>
> Subject: Re: 2.3.1 Replication is throwing scary errors
> Message-ID: <E7E7A927-68F8-443F-BA59-E66CED8FE878 at ellael.org>
> Content-Type: text/plain;	charset=utf-8
> 
> Reuben Farrelly wrote:
>> From: Michael Grimm <trashcan at ellael.org>
> 
>>> [This is Dovecot 2.3.1 at FreeBSD STABLE-11.1 running in two jails at distinct servers.]
>>> I did upgrade from 2.2.35 to 2.3.1 today, and I do become pounded by error messages at server1 (and vice versa at server2) as follows:
>>> 	| Apr  2 17:12:18 <mail.err> server1.lan dovecot: doveadm: Error: dsync(server2.lan): I/O has stalled, \
>>> 		no activity for 600 seconds (last sent=mail_change, last recv=mail_change (EOL))
>>> 	| Apr  2 17:12:18 <mail.err> server1.lan dovecot: doveadm: Error: Timeout during state=sync_mails \
>>> 		(send=changes recv=mail_requests)
>>> 	[?]
>>> 	| Apr  2 18:59:03 <mail.err> server1.lan dovecot: doveadm: Error: dsync(server2.lan): I/O has stalled, \
>>> 		no activity for 600 seconds (last sent=mail, last recv=mail (EOL))
>>> 	| Apr  2 18:59:03 <mail.err> server1.lan dovecot: doveadm: Error: Timeout during state=sync_mails \
>>> 		(send=mails recv=recv_last_common)
>>> I cannot see in my personal account any missing replications, *but* I haven't tested this thoroughly enough. I do have customers being serviced at these productive servers, *thus* I'm back to 2.2.35 until I do understand or have learned what is going on.
> 
> In my reply to this statement of mine I mentioned that I have seen those timeouts quite some times during the past year. Thus, I upgraded to 2.3.1 again, and boom: after some hours I ended up in hanging processes [1] like (see Remko's mail in addition) ...
> 
> 	doveadm-server: [IP4/6 <USER1> SOME/MAILBOX import:0/0] (doveadm-server)
> 
> ? at server2 paired with a file like ?
> 
> 	-rw------- 1 vmail dovecot uarch 0 Apr 3 16:52 /home/to/USER1/.dovecot-sync.lock
> 
> Corresponding logfile entries at server2 are like ?
> 
>    Apr  3 17:10:49 <mail.err> server2.lan dovecot: doveadm: Error: Couldn't lock /home/to/USER1/.dovecot-sync.lock: \
>    fcntl(/home/to/USER1/.dovecot-sync.lock, write-lock, F_SETLKW) locking failed: Timed out after 30 seconds \
>    (WRITE lock held by pid 51110)
> 
> [1] Even stopping dovecot will not end those processes. One has to manually kill those before restarting dovecot.
> 
> After one day of testing 2.3.1 with a couple of those episodes of locking/timeout, and now missing mails depending with server your MUA will connect to, I went back to 2.2.35. After two days at that version I never had such an episode again.
> 
>> It's not just you.  This issue hit me recently, and it was impacting
>> replication noticeably.  I am following git master-2.3 .
> [...]
>> There is also a second issue of a long standing race with replication
>> occurring somewhere whereby if a mail comes in, is written to disk, is
>> replicated and then deleted in short succession, it will reappear
>> again to the MUA.  I suspect the mail is being replicated back from
>> the remote.  A few people have reported it over the years but it's not
>> reliable or consistent, so it has never been fixed.
>> And lastly there has been an ongoing but seemingly minor issue
>> relating to locking timing out after 30s particularly on the remote
>> host that is being replicated to.  I rarely see the problem on my
>> local disk where almost all of the mail comes in, it's almost always
>> occurring on the replicate/remote system.
> 
> It might be time to describe our setups in order to possibly find common grounds that might trigger this issue you describe and Rimko and myself ran into as well.
> 
> Servers:	Cloud Instances (both identical), around 25ms latency apart.
> 		Intel Core Processor (Haswell, no TSX) (3092.91-MHz K8-class CPU)
> 		Both servers are connected via IPsec/racoon tunnels
> OS:		FreeBSD 11.1-STABLE (both servers)
> Filesystem:	ZFS
> MTA:		postfix 3.4-20180401 (postfix delivers via dovecot's LMTP)
> IMAP:		dovecot running in FreeBSD jails (issues with 2.3.1, fine with 2.2.35)
> Replication:	unsecured tcp / master-master
> MUA:		mainly iOS or macOS mail.app, rarely roundcube

For me:

Servers:	Main: VM on a VMWare ESXi local system (light
		load), local SSD disks (no NFS)
		Redundant: Linode VM in Singapore, around 250ms away
		Also no NFS.  Linode use SSDs for IO.
		There is native IPv6 connectivity between both VMs.
		As I am using TCPs I don't have a VPN between them -
		just raw IPv6 end to end.
OS:		Gentoo Linux x86_64 kept well up to date
Filesystem:	EXT4 for both
MTA:		Postfix 3.4-x (same as you)
IMAP:		Dovecot running natively on the machine (no jail/chroot)
Replication:	Secured TCP / master-master (but only one master is
		actively used for SMTP and MUA access)
MUA:		Thunderbird, Maildroid, MS Mail, Gmail client,
		Roundcube, LG Mail client on handset etc

> I believe it is worthwhile to mention here that I run a poor man's fail-over approach (round-robin DNS) as follows:
> 
> DNS:		mail.server.tld resolves to one IP4 and one IP6 address of each server, thus 4 IP addresses in total

For me I have a simple pair of servers that replicate mail to each 
other.  Clients connect to the main server only, which is where mail 
comes in.  The second one that experiences the problem with locking is 
only a redundant server which is a secondary MX, that only gets 
occasional SMTP deliveries from time to time.  So the amount of 
replication back from the "secondary" is very minimal; the vast bulk of 
the replication is the primary replicating changes out.

Reuben