Re: [PATCH 4/7] megaraid_sas: Send SYNCHRONIZE_CACHE command to firmware

["Ewan D. Milne" <emilne@redhat.com>]

On Mon, 2016-10-17 at 12:08 -0400, Ric Wheeler wrote:
> On 10/17/2016 11:55 AM, Christoph Hellwig wrote:
> > On Mon, Oct 17, 2016 at 09:01:29AM -0400, Ric Wheeler wrote:
> >> This must go in - without this fix, there is no data integrity for any file system.
> > megaraid always had odd ideas on cache flushing, and this might be
> > a opportunity to write down all the assumptions and document them.
> >
> >> In effect, this driver by default has been throwing away SYNCHRONIZE_CACHE
> >> commands even when acting in JBOD/non-RAID mode.
> > That would explain some issues we've seen with megaraid hardware, but
> > it seems a bit too shocking to be true.
> >
> > Looking over the patch I disagree with the module option - we must do
> > the right thing by default, which is sending SYNCHRONIZE_CACHE commands
> > if the WCE bit is set.  If there are controllers where this is harmful
> > for RAID mode and we can't fix the firmware in time we'll need to make
> > special exceptions for this case in the driver based on the PCI ID
> > and knowing what we talk to instead of leaving it to the user.
> 
> I do agree - having users be able to disable this easily is asking for trouble. 
> It will be slower, but slower because the cache flush actually is effective.

Absolutely.  It is a bad idea to allow users to disable correct behavior
in order to achieve better performance, you will always get people who
will do this and then pay the price eventually when they get corruption.

> 
> >
> >> * having T10 & T13 report the existence of a volatile write cache - this is
> >> different than WCE set, some devices have a write cache and are
> >> battery/flash backed.
> > T10 is pretty clear now the WCE should only be set for a non-voltile
> > cache.  For a while they had odd NV bits to allow flushing a
> > non-volatile cache, but in the latest revisions all that is gone.
> 
> If T10 has clarity on this, then the actual fix would be to have the driver 
> advertise WCE enabled only for the pass through/non-RAID luns (assuming the 
> drive's write cache was enabled) and then we would leave WCE disabled for the 
> targets that the firmware handles cache destaging for.
> 
>