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? -- This message posted from opensolaris.org
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
Erik.Trimble at Sun.COM said:> 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.Docs for the non-RAID HBA''s sold by Sun say that with proper (recent) firmware, at power-up the HBA will disable write caches on the disks themselves (this refers to the LSI 1068-based HBA''s, anyway). There was a Sun Alert issued for early revisions which failed to disable disk caches, resulting in data loss at power loss for cache-unaware software. Solaris/OpenSolaris ZFS will then enable the write caches once it knows it has control of whole disks, and issues flushes to the drives as appropriate. Regards, Marion
Marion - Do you happen to know which SAS hba it applys to? -- This message posted from opensolaris.org
beneri3 at yahoo.com said:> Marion - Do you happen to know which SAS hba it applys to?Here''s the article: http://sunsolve.sun.com/search/document.do?assetkey=1-66-248487-1 The title is "Write-Caching on JBOD SATA Drive is Erroneously Enabled by Default When Connected to Non-RAID SAS HBAs". By the way, you can use "raidctl" to view/manage firmware on these. Regards, Marion