gjournal and calculation of the size of journal provider

[Miroslav Lachman]

What is the right rule for journal size calculation?
There are two sources stating different things.

1] journal size depends on disk write speed
http://lists.freebsd.org/pipermail/freebsd-fs/2006-June/002016.html

"For example your disk can write
at 60MB/s. Journal switch time is 10 seconds. The journal provider has
to have place to keep two journals (active and inactive). So bascially
you need 60*10*2MB + gjournal headers."


2] journal size depends on RAM size
http://www.freebsd.org/doc/en/articles/gjournal-desktop/article.html#UNDERSTANDING-JOURNALING

"Your RAM size should fit in 30% of the journal provider's space. For 
example, if your system has 1 GB RAM, create an approximately 3.3 GB 
journal provider. (Multiply your RAM size with 3.3 to obtain the size of 
the journal)."


What's the right size for journal on 143GB 15k rpm SAS disks on machine 
with 16GB of RAM? Based on second case, it will be more than 50 GB - one 
third of the size of disk. This is insane vasting.

I have gjournal on few of our machines with size of journal set to 2GB 
on SATA disks in gmirror.

Miroslav Lachman