$Id: FAQ.Capacity_Planning.txt,v 1.3 2022/03/22 11:12:25 gilles Exp gilles $

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https://imapsync.lamiral.info/FAQ.d/FAQ.Capacity_Planning.txt


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            Imapsync tips for Capacity Planning.
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I plan to go to a distributed architecture for the online service. I'm
doing some capacity planning. Imapsync takes memory, cpu and bandwidth
in a relatively deterministic values.

My current question is: Shall I use 
* N   2GB hosts
* N/2 4GB hosts
* N/4 8GB hosts

Let's do some maths

The CPU can be an issue. On average, an imapsync run takes 5% of the
overall cpu time for a Intel i5-2300 with 4 cores.  It implies 20
imapsync runs is ok on the current online host before the cpus become
the bottleneck.  As a rule of thumb, imapsync takes 20% of a cpu core.

The RAM can be an issue. On average, an imapsync run takes 250 MB.  So
4 imapsync processes per GB is the limit before swapping to disk,
which is a known phenomenum telling when memory becomes the bottleneck.
16 GB allows 64 imapsync processes.

The Bandwidth I/O can be an issue.  The "Average bandwidth rate" value
given by imapsync at the end of a transfer, and also the bandwidth
rate given during the sync on the ETA line, is the total size of all
messages copied divided by the time passed. If imapsync is run between
two foreign imap servers then the total size transferred on the
network link is twice this value, one time when getting the message
from host1, rx, and one time sending the message to host2, tx.

On average, an imapsync runs at 3 Mbps both ways, rx and tx, so 6 Mbps
in total.  So a 100 Mbps symetric link allows 33 imapsync processes
before the link becomes the bottleneck.

The best minutes observed so far are a global 187 Mpbs rate (23 MiB/s) 
on a 100 Mbps symetrical link done by 28 imapsync processes in parallel, 
6.7 Mbps per process at that minute.  But the best minutes observed for a
single imapsync process is at 103 Mbps, the rx/tx link limit.


The harddisk I/O can be an issue. I don't measure it yet because
imapsync doesn't perform heavy I/0 where it runs. Well, I don't know,
no measure is no knowledge, just guesses.