ZFS on backup fileserver - RAM usage

Jeremy Chadwick koitsu at FreeBSD.org
Mon Oct 13 12:38:26 UTC 2008

On Mon, Oct 13, 2008 at 01:44:32PM +0200, Miroslav Lachman wrote:
> I am planning to install new server for backups with 4x 1TB SATA II  
> drives in RAIDZ. There will be about 20 separated filesystems from one  
> zpool, few jails with ssh (scp/sftp), rsync and maybe FTP daemons, no  
> other services with huge RAM utilization. As FreeBSD 7.1(-BETA) amd64  
> still have some limits of kernel space memory, are there any benefits to  
> put more then 2GB or 3GB in this server? Will it be more stabel or  
> faster with for example 6GB of RAM? (I can buy it, RAM is really cheap  
> in these days, but will it have some sense or is it vaste?)

Adding more RAM will work just fine for userland programs, meaning they
will be able to make use of the additional RAM.  The kernel, with
regards to kmap and kmem, however, will not.  If you need that
functionality, you'll have to run CURRENT.

> I am using this tuning on testing machine (with 2GB RAM):
> vm.kmem_size="1024M"
> vm.kmem_size_max="1024M"
> vfs.zfs.prefetch_disable="1"
> vfs.zfs.arc_min="16M"
> vfs.zfs.arc_max="64M"
> kern.maxvnodes="400000"
> (recommendations from http://wiki.freebsd.org/ZFSTuningGuide)
> Have somebody better results with another values?

The values look fine, but keep in mind that you still may encounter
crashing with that kind of load (you're sticking a lot of stuff on one
single box, all of which utilises ZFS heavily).

You'll simply need to tune these as those situations arise.

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

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