Thinking of using ZFS/FBSD for a backup system

[Kris Kennaway]

Jeremy Chadwick wrote:
> On Wed, Jul 09, 2008 at 07:44:20AM +0200, CZUCZY Gergely wrote:
>> On Wed, 09 Jul 2008 01:14:49 +0200
>> Kris Kennaway <kris at FreeBSD.org> wrote:
>>
>>> CZUCZY Gergely wrote:
>>> I don't know; empirically my setup is an upper bound.  How large was
>>> "as large as it was allowed" for you?
>> Well, we cannot buy "upper bounds" all over, just because some
>> developer is unable to figure out things. I think you can't expect
>> FreeBSD users to spend as much money as possible, just because the devs
>> can't tell how much is enough...
>> It seems more like a twilight zone then a stable feature now ;)
>>
>> It was exactly as much as an amd64 installation would allow with 2GB of
>> physical memory. We've dismissed the setup around february, and I don't
>> have the configs anymore. It was an amd64 setup with 2GB of physical
>> memory.
> 
> The bottom line here is that i386 and amd64 both have a kmem_size limit
> of 2GB.

No, it's the limit on KVA (address space), not kmem_size.  On i386 there 
is only 1GB of KVA by default, so it's even harder to fit ZFS in.  I 
thought you could tune it higher than 2GB if you liked, although this 
comes out of address space available to user programs (4GB total for 
user + kernel).

> You can throw 32GB of RAM into an amd64 box, but FreeBSD will
> only utilise up to 2GB of that for kmem.  That is purely a FreeBSD
> limitation, and is being dealt with in HEAD by Alan Cox.  I believe he
> has a patch, or it may have been committed -- I don't follow HEAD.  I
> can point people to a mailing list URL, if needed.
> 
> This is one of the limitations Gergely is referring to.

No it's not, since he has only 2GB of physical memory.

> Since ZFS is incredibly memory-hungry, you're forced to tune ZFS to try
> and get it to "play nice" with that 2GB limit on STABLE/RELEASE systems.
> You also need to keep in mind that you can't just set kmem_size and
> kmem_size_max to 2048M, because the kernel needs memory for other
> things.
> 
> The tuning parameters I use on my 2GB amd64 and 4GB amd64 boxes are:
> 
> vm.kmem_size="1536M"
> vm.kmem_size_max="1536M"
> vfs.zfs.arc_min="16M"
> vfs.zfs.arc_max="64M"
> 
> If you set kmem_size and kmem_size_max any higher than that, the machine
> will panic on boot, stating (indirectly) that there isn't enough memory
> available for the kernel to allocate for other things.

Yes, I said this earlier :)

> Until I added the arc_min and arc_max setting, I could occasionally
> panic the machines under very heavy load (heavy zpool I/O), caused by
> kmem exhaustion.  Since adding the arc_* tunings, I've tried very hard
> to crash the machines, and I cannot.

Good to hear.

> But there's absolutely no guarantee those tuning parameters above will
> ensure FreeBSD won't panic due to kmem exhaustion.  I believe this is
> the point Gergely is making about the "stability" of the whole thing.

Not having the resources to run a very memory-intensive filesystem does 
not make it "unstable", it makes it "too memory intensive".

Kris