On May 17, 2012, at 12:19 PM, Paynter, Richard wrote:

> I have a customer who wants to set zfs_arc_max to 1G for a 16G system, and 2G 
> for a 32G system.  Both of these are SAP and/or Oracle db servers.  They 
> apparently want to maximize the amount of memory available for the 
> applications.

This is the best use case of the zfs_arc_max tunable.

> My question, now, is whether it would be advisable to do this, and – if not – 
> what the impact might be.

For this use case, it is a good idea. The impact of not setting it occurs when 
you later
want to start a memory-hogging program, the request for arc to shrink can take 
a little
while to satisfy, and the memory-hog might not be designed to cope with 
retrying large
memory allocation requests.

> The 32G system is an M3000; the 16G system is a V490.  Both are running 
> Solaris 10 Update 8, patched to 147440-09.
> I have a feeling that setting zfs_arc_max this low is not a good idea, but 
> would like to get some corroborating information/references, if possible.

This use case is why the zfs_arc_max tunable exists.
 -- richard

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