SAS Raid - mfi driver

Fredrik Widlund fredrik.widlund at
Thu Nov 2 09:50:20 UTC 2006

Because the card itself will deal with the buffered writes independently
of the kernel activity the risk should be less than using softupdates.
Words like "screwed" seems to me to be exaggerated in the generic case.
I our case specific you would need to understand the nature of what we
are doing to be able to make a comment like that. For example data is
redundant (exists in many copies), consists of very large sequencial
files, we have plenty of backup power, and the greatest risk is fbsd
locking up/crashing.

Anyway our specific case is not of interest here, I just wanted to share
our experiences with the LSI MegaSAS with other fbsd users so they
understand why they get a severe performance degradation if they try to
use such a card w/o a bbu, and what their options are.

The generic case of how great the risk really is of corrupting
filesystems completely using caches of any kind on the way to secondary
storage still is interesting to me, so if you could elaborate here that
would be great!

Kind regards,
Fredrik Widlund

Bob Willcox wrote:
> On Wed, Nov 01, 2006 at 12:34:22AM +0100, Fredrik Widlund wrote:
>> Yes, it forces writeback even when the controller has no BBU. Choosing 
>> WBack itself will default back to WThru. It's dangerous, but I guess it 
>> should be much less dangerous than using for example softupdates.
> I don't see how it could be *less* dangerous than using softupdates. Any
> loss of power while writing and it seems to me that you are going to be
> screwed w/o a BBU.
> [snip]

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