[BlueOnyx:24793] Re: IMAP resources

[Roy Urick]

Thank you for the insight.

On 2/23/2021 6:16 PM, Michael Stauber wrote:
> Hi Roy,
>
>> Is there any performance difference on the server side between moving
>> messages to trash and flat out deleting them via an IMAP connection? If
>> so which is easier on the server? Or is there even a performance
>> difference?
> There are a couple of factors that play a role there and it also depends
> on what IMAP implementation they use (Dovecot, UW-IMAP or something
> else). But we can generalize a bit for that purpose.
>
> In essence the performance of that transaction depends on available
> processing power, possible I/O speed given whatever else the system is
> doing and the storage format in which the emails are stored.
>
> Take a run of the mill Dovecot install where the inbox is in Mbox
> format. The folder "Trash" is a separate folder with another flat file
> in it that contains the Trash-messages.
>
> Say there are a couple of thousand emails in the Inbox and the email you
> want to move to trash is near the end of that monolithic file, but not
> the last message.
>
> The IMAP daemon now needs to open up the large monolithic Mbox file that
> contains your inbox and it has to splice out the message you want to
> move to trash. It has to read the entire file into memory, has to remove
> the message from it and then write the changed Mailbox file back to
> disk. Then it needs to access the equally monolithic Trash Mbox file and
> needs to append the message to the end. This can be done by
> concatenating, which is less resource intense, but how it's done largely
> depends on the IMAP daemon.
>
> So this whole transaction from start to finish will be *much* more
> resource intensive the larger these monolithic mailbox files are.
>
> If you use Maildir format (instead of Mbox), then this is a lot less
> resource intense, as emails are stored in individual files. Therefore
> transactions such as read, write or move are generally less resource
> intense and faster, as the process doesn't have to juggle a massive Mbox
> file around.
>
> Still: If you have many emails in Maildir format you introduce another
> bottleneck, which is directory indexing. Which can also be resource
> intensive, but on a different level and generally it's still somewhat
> less taxing that lobbing a +4GB Mbox file around on each access to emails.
>
> But like said: This is just one (likely) scenario as I don't know what
> they're using. You can also use Dovecot with an SQL backend for message
> storage, in which case all these assumptions fly out of the window.
>