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.
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