Re: [dm-devel] [PATCHSET block#for-2.6.36-post] block: replace barrierwith sequenced flush

[Ric Wheeler]

On 08/23/2010 12:45 PM, Sergey Vlasov wrote:
> On Mon, Aug 23, 2010 at 11:19:13AM -0400, Ric Wheeler wrote:
> [...]
> > (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.
>
> Actually some of such cards keep write cache on the drives enabled and
> issue FLUSH CACHE commands to the drives.  E.g., 3ware 9690SA behaves
> like this at least with SATA drives (the FLUSH CACHE commands can be
> seen after enabling performance monitoring - they often end up in the
> "10 commands having the largest latency" table).  This can actually be
> safe if the card waits for the FLUSH CACHE completion before making
> the write cache data in its battery-backed memory available for reuse
> (and the drive implements the FLUSH CACHE command correctly).

Yes - this is certainly one way to do it. Note that this will not work if the 
card advertises itself as a write through cache (and we end up not sending down 
the SYNC_CACHE commands).

At least one hardware RAID card (I unfortunately cannot mention the brand) did 
not do this command forwarding.

ric

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