[zfs-discuss] Re: Memory Usage

johansen
Tue, 12 Sep 2006 21:58:57 -0000

> 1) You should be able to limit your cache max size by
> setting arc.c_max.  Its currently initialized to be
> phys-mem-size - 1GB.

Mark''s assertion that this is not a best practice is something of an
understatement.  ZFS was designed so that users/administrators wouldn''t
have to configure tunables to achieve optimal system performance.  ZFS
performance is still a work in progress.

The problem with adjusting arc.c_max is that its definition may change from one
release to another.  It''s an internal kernel variable, its existence
isn''t guaranteed.  There are also no guarantees about the semantics of
what a future arc.c_max might mean.  It''s possible that future
implementations may change the definition such that reducing c_max has other
unintended consequences.

Unfortunately, at the present time this is probably the only way to limit the
cache size.  Mark and I are working on strategies to make sure that ZFS is a
better citizen when it comes to memory usage and performance.  Mark has recently
made a number of changes which should help ZFS reduce its memory footprint.
However, until these changes and others make it into a production build
we''re going to have to live with this inadvisable approach for
adjusting the cache size.

-j


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