On Mittwoch 04 Februar 2009 Ralf Liebenow wrote:
> Should Battery backed RAID controllers not always set their discs
> cache off ?
> As I see it (in case of a power failure):
> - the discs are connectet to the main power, so if there is a power
> failure they're offline at that moment in time and their (write)
> cache will be gone in that instance of time too
Normally a server is on a UPS, and that should report when there's a
power outage so the server has enough time to gracefully shut down.
Still, there can be other events such as:
- power supply error. Even with redundant PS, an outage can exist
- human error (coffee into the server, someone unplugging the cable
between UPS and server,...)
- and of course mainboard/cpu/ram total crashes
so you are basically never safe.
> - if a RAID controller does not turn off the disks write cache, the
> controller cannot know if previous writes have made it to the disk.
The controller could keep in-transfer blocks in it's cache, waiting for
a confirm from the disk that the blocks are on the media, and only
afterwards remove it from cache. I don't know if controllers do that
actually. I'll ask Areca support on that.
> good RAID Controller would also use its cache to re-organise the disc
> writes to minimize seek times doing somthing like intelligent command
> queuing. This would also mean, that any order of writes to a disk
> could have been changed by the controller. This would ultimately
> break any filesystem which does not explicitly fsyncing consistent
> checkpoints to the disk, which would make battery backed RAID Systems
> pretty useless ... would it ?
> So .. a battery backed RAID controller should default to "no disk
> write cache" should it ? Otherwise why should anyone want to use such
> expensive controllers ... it just does not make sense to have a
> battery backed cache on the controller, when things get inconsistent
> at a power outage ... It wouldn't have any purpuse ... I hope
> developers of battery backed RAID controllers are aware of that
> implication ...
Yes, imagine you have a RAID with 8 hard disks each having 32MB cache...
up to 256MB data lost, with a very big chance of having filesystem
metadata in cache, as that's written very often...
I'll be back on that once I have an official answer from Areca.
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