LSI MegaRAID SAS 9240 with mfi driver?

[Jan Mikkelsen]

On 31/03/2012, at 9:21 AM, Doug Ambrisko wrote:

> Jan Mikkelsen writes:
> | I don't know what changes Sean did. Are they in 9.0-release, or do I 
> | need -stable after a certain point? I'm assuming I should be able to 
> | take src/sys/dev/mfi/... and src/usr.sbin/mfiutil/... from -current.
> 
> It's in the SVN project/head_mfi repro.  You can browse it via the web at:
> 	http://svnweb.freebsd.org/base/projects/head_mfi/
> 
> It's not in -current yet.  I'm working on the.  I just did all the
> merges to a look try and eye'd them over.  Now doing a compile test
> then I can check it into -current.

OK, will check it out.

> | The performance is an interesting thing. The write performance I care 
> | about is ZFS raidz2 with 6 x JBOD disks (or 6 x single disk raid0) on 
> | this controller. The 9261 with a BBU performs well but obviously costs more.
> 
> There will need to be clarification in the future.  JBOD is not that
> same as a single disk RAID.  If I remember correctly, when doing some
> JBOD testing version single disk RAID is that JBOD is slower.  A 
> single disk RAID is faster since it can use the RAID.  However, without
> the battery then you risk losing data on power outage etc.  Without the
> battery then performance of a JBOD and single disk RAID should be able
> the same.
> 
> A real JBOD as shown by LSI's firmware etc. shows up as a /dev/mfisyspd<n>
> entries.  JBOD by LSI is a newer thing.

Ok, interesting. I was told by the distributor that the 9240 supports JBOD mode, but the 9261 doesn't. I'm interested to test it out with ZFS.

> 
> | I can see the BBU being important for controller based raid5, but I'm 
> | hoping that ZFS with JBOD will still perform well. I'm ignorant at this 
> | point, so that's why I'm trying it out. Do you have any experience or 
> | expectations with a 9240 being used in a setup like that?
> 
> The battery or NVRAM doesn't matter on the RAID type being used since the
> cache in NVRAM mode, says done whenever it has space in the cache for the
> write.  Eventually, it will hit the disk.  Without the cache working in
> this mode the write can't be acknowledged until the disk says done.  So
> performance suffers.  With a single disk RAID you have been using the
> cache.

With RAID-5 it is important because a single update requires two writes and a failure in the window where one write has completed and one write has not could cause data corruption. I don't know whether the controller really handles this case.

I guess I'm hopeful that ZFS will perform the function performed by the NVRAM on the controller. I can see how the controller in isolation is clearly slower without a BBU because it has to expose the higher layers to the disk latency.

> Now you can force using the cache without NVRAM but you have to acknowledge
> the risk of that.

Yes, I understand the risk, and it is one I do not want to take. All the 9261s I have deployed have a BBU and go into write through mode if the battery has a problem.

I think I need to test it in the context of ZFS and see how it works without controller NVRAM.

Regards,

Jan.