zfs arc and amount of wired memory

Jeremy Chadwick freebsd at jdc.parodius.com
Wed Feb 8 20:50:03 UTC 2012

On Wed, Feb 08, 2012 at 10:29:36PM +0200, Andriy Gapon wrote:
> on 08/02/2012 12:31 Eugene M. Zheganin said the following:
> > Hi.
> > 
> > On 08.02.2012 02:17, Andriy Gapon wrote:
> >> [output snipped]
> >>
> >> Thank you.  I don't see anything suspicious/unusual there.
> >> Just case, do you have ZFS dedup enabled by a chance?
> >>
> >> I think that examination of vmstat -m and vmstat -z outputs may provide some
> >> clues as to what got all that memory wired.
> >>
> > Nope, I don't have deduplication feature enabled.
> OK.  So, did you have a chance to inspect vmstat -m and vmstat -z?


Politely -- recommending this to a user is a good choice of action, but
the problem is that no user, even an experienced user, is going to know
what all of the "Types" (vmstat -m) or "ITEMs" (vmstat -z) correlate
with on the system.

For example, for vmstat -m, the ITEM name is "solaris".  For vmstat -z,
the Types are named zio_* but I have a feeling there are more than just
that which pertain to ZFS.  I'm having to make *assumptions*.

The FreeBSD VM is highly complex and is not "easy to understand" even
remotely.  It becomes more complex when you consider that we use terms
like "wired", "active", "inactive", "cache", and "free" -- and none of
them, in simple English terms, actually represent the words chosen for
what they do.

Furthermore, the only definition I've been able to find over the years
for how any of these work, what they do/mean, etc. is here:


And this piece of documentation is only useful for people who understand
VMs (note: it was written by Matt Dillon, for example).  It is not
useful for end-users trying to track down what within the kernel is
actually eating up memory.  "vmstat -m" is as best as it's going to get,
and like I said, with the ITEM names being borderline ambiguous
(depending on what you're looking for -- with VFS and so on it's spread
all over the place), this becomes a very tedious task, where the user or
admin have to continually ask developers on the mailing lists what it is
they're looking at.

| Jeremy Chadwick                                 jdc at parodius.com |
| Parodius Networking                     http://www.parodius.com/ |
| UNIX Systems Administrator                 Mountain View, CA, US |
| Making life hard for others since 1977.             PGP 4BD6C0CB |

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