Quote:> And milliseconds count, in fact: it means that every time the system
> does a write it needs to wait until the write is safely on disk if it
> wants to be safe, which perhaps takes several ms, which might easily be
> more than the time to actually write the data.
If PC's were designed well it wouldn't have an influence because the
condensator in your power supply are able to keep the power for a few
hundert milliseconds. Enough time for the disk to write it, but the OS
does not know that it must immediately stop queuing new data.
Quote:> SCSI actually is much smarter, because the tagged-queuing stuff lets
> the system have several writes in flight at once to the disk, with the
> disk notifying when they complete, so it can often hide the latency,
> without needing to rely on an unreliable write-cache (it's more-or-less
> the same trick that processors do to hide latency). I think that SATA
> disks (and maybe just plain ATA ones) can do this now too.
I was thinking about one of the SCSI features that allows the
controller to map bad blocks to different locations on the disk without
notifying the OS about this change (because in mid 90 the OS didn't do
anything usefull with this information).
So even when the OS thinks that blocks are close together SCSI disks
might need a long way to reach the block (and killing the elevator
algorithm in the OS). I hope that this SCSI feature is now disabled by
In this scenario we have hunderts of milliseconds in the worst case.