[dm-devel] [PATCHSET block#for-2.6.36-post] block: replace barrier with sequenced flush
Ric Wheeler
rwheeler at redhat.com
Mon Aug 23 16:49:54 UTC 2010
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
More information about the dm-devel
mailing list