Could not connect to socket /var/imap/socket/lmtp: Connection refused by localhost

[Simon Matter]

> Hi.
>
> On 17.01.2017 19:09, Andy Dorman via Info-cyrus wrote:
>>
>> I am not an expert by any means and I hope someone corrects me if I
>> make a bad suggestion...but I have two questions:
>>
>> 1. It sounds like you have a heavily used server, so why do you have
>> Cyrus listening on both "localhost:lmtp" AND a unix socket
>> "/var/imap/socket/lmtp"?
>>
>> From the log entry it looks like your MTA uses a unix socket. Unless
>> you have something else (mail clients or other MTAs running on your
>> Cyrus server?) that need to communicate via the localhost:lmtp port,
>> you could comment out the unneeded lmtp service line and save those
>> resources.
> Well, on one hand you are right, seems like noone uses network lmtp
> connections, but on the other hand how can the idle processes save
> resources ? They only can save the memory, which doesn't seem to be the
> problem. However, I will try you advice.
>>
>> 2. You say "increasing this value can make the situation even worse".
>> Which value?  There are 5 values on those two lines that you could
>> increase.  And by "even worse" do you mean even more refused
>> connections?
> The maxchild number.
>>
>> While I am not a Cyrus guru, I have seen my share of overloaded mail
>> servers and if you are running into a disk IO limit, adding more
>> processes fighting over a limited resource is very likely to make
>> things worse.  So you should also confirm a hardware limitation is not
>> at play here.
> Yup, this is exaclty what happens when increasing the maxchild number:
> more messages start to bounce. And yes, the disks iops seems to be the
> limiting factor. So, are there any other approaches besides scaling out
> the disks iops ?

I remember a situation more than a decade ago where we had to tune this.
The problem with LMTP deliveries was that a lot of mails with hundreds of
recipients on the same server were sent. Our Postfix MTA has sent every
mail by a single LMTP transfer which resulted in high LMTP load and
prevented effective usage of single instance store. The problem was solved
by setting "local_destination_recipient_limit = X00" on the Postfix MTA (I
don't remember the exact number).

Regards,
Simon