On Sat, Oct 25, 2003 at 11:04:00PM -0700, James Moe wrote:
> Other posts have noted that SCSI never fails under this condition. Apparently SCSI
> drives sense an impending power loss and flush the cache before power completely
> disappears. Speed *and* reliability. Hm.
I understood it differently. Postgresql has WAL to deal with this situation.
This issue that it only works as long as the drive doesn't lie about which
blocks have been written and which are merely in cache. Apparently IDE disks
lie and SCSI disks don't. It may be a protocol thing.
The other alternative is battery backed memory. i.e. keep the blocks in
memory hoping that power will return to the drive before it fails. Some RAID
cards do this.
Another thing is that 3ware RAID controllers stick a SCSI interface in
front of the IDE drives, so perhaps it has more scope to deal with this
issue.
Remember, when power fails the first thing that happens is the system
cancels any DMA tranfer in progress as memory is the part most sensative to
power fluctuations.
> Of course, anyone serious about a server would have it backed up with a UPS and
> appropriate software to shut the system down during an extended power outage. This just
> leaves people tripping over the power cords or maliciously pulling the plugs.
If you start adding up the points of failure it's quite a lot. But you
should be able to proof the system against even malicious tampering.
--
Martijn van Oosterhout <kleptog@svana.org> http://svana.org/kleptog/
> "All that is needed for the forces of evil to triumph is for enough good
> men to do nothing." - Edmond Burke
> "The penalty good people pay for not being interested in politics is to be
> governed by people worse than themselves." - Plato