Open Stack

On 09/15/2015 01:16 AM, Clay Gerrard wrote:
> Idk about a chestnut, but there's this:
>
> http://lists.openstack.org/pipermail/openstack-operators/2011-October/000297.html

That advice isn't quite accurate.  It says "Enabling barriers effectively turns 
all writes into Write-Through operations, so the write goes straight to the disk 
platter and you get little performance benefit from the raid card (which hurts a 
lot in terms of lost iops)."

That's actually not necessarily true.  It's complicated (See 
http://marc.info/?l=linux-scsi&m=136674609728515&w=2 for discussion) but from 
what I understand most hardware RAID controller vendors will not actually flush 
their non-volatile cache on a SCSI "SYNCHRONIZE CACHE" command.  They will 
ensure that the data is in the NV cache, and then return to the host.  (Which 
should be quick.)

This gets you the best of both worlds...you get the reliability of filesystem 
barriers enforcing ordering through the OS stack, with the performance benefit 
of not having to flush the data all the way down to the disks.

Chris