Re: [PATCHSET block#for-2.6.36-post] block: replace barrier with sequenced flush

[Ric Wheeler]

On 08/23/2010 10:01 AM, Jens Axboe wrote:
> On 2010-08-23 15:58, Ric Wheeler wrote:
> > On 08/23/2010 08:48 AM, Christoph Hellwig wrote:
> > > On Mon, Aug 23, 2010 at 02:30:33PM +0200, Tejun Heo wrote:
> > > > It might be useful to give several example configurations with
> > > > different cache configurations.  I don't have much experience with
> > > > battery backed arrays but aren't they suppose to report write through
> > > > cache automatically?
> > >
> > > They usually do.  I have one that doesn't, but SYNCHRONIZE CACHE on
> > > it is so fast that it effectively must be a no-op.
> >
> > Arrays are not a problem in general - they normally have internally, redundant
> > batteries to hold up the cache.
> >
> > The issue is when you have an internal hardware RAID card with a large cache.
> > Those cards sit in your server and the batteries on the card protect its
> > internal cache, but do not have the capacity to hold up the drives behind it.
> >
> > Normally, those drives should have their write cache disabled, but sometimes
> > (especially with S-ATA disks) this is not done.
>
> The problem purely exists on arrays that report write back cache enabled
> AND don't implement SYNC_CACHE as a noop. Do any of them exist, or are
> they purely urban legend?


Hi Jens,

There are actually two distinct problems:

(1) arrays with a non-volatile write cache (battery backed, navram, whatever) 
that do not NOOP a SYNC_CACHE command. I know of one brand that seems to do 
this, but it is not a common brand. If we do not issue flushes for write through 
caches, I think that we will avoid this in any case.

(2) hardware raid cards with internal buffer memory and on-card battery backup 
(they sit in your server, disks sit in jbod like expansion shelves). These are 
fine if the drives in those shelves have write cache disabled.

ric
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