On Wed, 26 Dec 2007, david@lang.hm wrote:
> yes, the two linux software implementations only read from one disk, but I
> have seen hardware implementations where it reads from both drives, and if
> they disagree it returns a read error rather then possibly invalid data (it's
> up to the admin to figure out which drive is bad at that point).
Right, many of the old implementations did that; even the Wikipedia
article on this subject mentions it in the "RAID 1 performance" section:
http://en.wikipedia.org/wiki/Standard_RAID_levels
The thing that changed is on modern drives, the internal error detection
and correction is good enough that if you lose a sector, the drive will
normally figure that out at the firmware level and return a read error
rather than bad data. That lowers of the odds of one drive becoming
corrupted and returning a bad sector as a result enough that the overhead
of reading from both drives isn't considered as important. I'm not aware
of a current card that does that but I wouldn't be surprised to discover
one existed.
--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD