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
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