Brad wrote:
Since the j4500 doesn't have a internal SAS controller, would it be safe to say
that ZFS cache flushes would be handled by the host's SAS hba?
Wellllll. It depends on what you mean as "cache flush". Cache flushes
happen at a couple of points:
(1) ZFS decides it's time to write to disk it's accumulated IO.
(2) RAID or other cache-holding controller does a write-out of it's
local cache
(3) The physical device (HD or SSD) clears its internal buffer
Now, nominally, when #1 happens, ZFS tries to tell #2 and #3 that they
too should initiate their own housecleaning then. Doesn't always
happen, though, as some controllers or HD/SSDs will ignore external
cache flush requests. Depending on the controller, it may or may not
also request the HD/SSDs flush their cache when it decides to do its own
cache flush. Which may or may not be obeyed by the HD/SSD, yet again.
All J4xxx systems are really nothing more than huge SAS expanders hooked
to a bunch of disks, so cache flush requests will either come from ZFS
or any attached controller. Note that I /think/ most non-RAID
controllers don't initiate their own cache flush requests.
--
Erik Trimble
Java System Support
Mailstop: usca22-123
Phone: x17195
Santa Clara, CA
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