Greg Smith wrote:
some write cache in the SATA disks...Since all non-battery backed caches need to get turned off for reliable database use, you might want to double-check that on the controller that's driving the SATA disks.
Is this really true? Doesn't the ATA "FLUSH CACHE" command (say, ATA command 0xE7) guarantee that writes are on the media? http://www.t13.org/Documents/UploadedDocuments/technical/e01126r0.pdf "A non-error completion of the command indicates that all cached data since the last FLUSH CACHE command completion was successfully written to media, including any cached data that may have been written prior to receipt of FLUSH CACHE command." (I still can't find any $0 SATA specs; but I imagine the final wording for the command is similar to the wording in the proposal for the command which can be found on the ATA Technical Committee's web site at the link above.) Really old software (notably 2.4 linux kernels) didn't send cache synchronizing commands for SCSI nor either ATA; but it seems well thought through in the 2.6 kernels as described in the Linux kernel documentation. http://www.mjmwired.net/kernel/Documentation/block/barrier.txt If you do have a disk where you need to disable write caches, I'd love to know the name of the disk and see the output of of "hdparm -I /dev/sd***" to see if it claims to support such cache flushes. I'm almost tempted to say that if you find yourself having to disable caches on modern (this century) hardware and software, you're probably covering up a more serious issue with your system.
