kern.geom.journal.stats.low_mem refers to what?

[Luke Dean]

I've recently set up gjournal on top of gmirror on FreeBSD 8.2.  I 
understand that this setup has a lot of redundant writing.  It is 
working, but I'm not sure I've set it up as efficiently as I should.

During prolonged writes, such as copying large files to the file system 
across the network or producing very large logfiles, the "low_mem" and 
"skipped_bytes" statisics rise rapidly and the system becomes less 
responsive.  "top" always reports free memory, so I don't think that's the 
issue.  "journal_full" and "wait_for_copy" have never occurred.  Here's a 
sample of what happens after a couple hours of intense writing...

kern.geom.journal.stats.low_mem: 5379
kern.geom.journal.stats.journal_full: 0
kern.geom.journal.stats.wait_for_copy: 0
kern.geom.journal.stats.switches: 7543
kern.geom.journal.stats.combined_ios: 265318
kern.geom.journal.stats.skipped_bytes: 935712768

"low_mem" sounds like a bad thing.  What could I do to remedy that?  Did I 
make the journal too small?  The stats say that "journal_full" has never 
happened, so maybe not?  Is there a setting I should tweak?

The Handbook says 1GB is good enough most of the time, but it also says 
that 3x the amount of physical memory is a good size as well.  I 
compromised between the two and made an 8GB journal for this system that 
has ~4GB of memory.