Re: Benefits from computing physical IDE disk geometry?

[Nick Piggin <piggin@cyberone.com.au>]

Chuck Ebbert wrote:

>>If RAID1 can use the generic elevator then it should. I
>>guess it can't though.
>>
>
>
>  No, but it is feeding IO requests into the elevators of the 
>block devices below it.  For a given read, all it wants to do
>is pick one device to handle the work.  If it could look into
>the queues maybe it could make better decisions.
>
OK right. As far as I can see, the algorithm in the RAID1 code
is used to select the best drive to read from? If that is the
case then I don't think it could make better decisions given
more knowledge. It really wants to know if the disk head is
close to request x however it is impossible to tell where the
disk head will be by the time request x is the next in line
for that disk, regardless if it can look at the low level
queues or not.

It seems to me that a better way to layer it would be to have
the complex (ie deadline/AS/CFQ/etc) scheduler handling all
requests into the raid block device, then having a raid
scheduler distributing to the disks, and having the disks
run no scheduler (fifo).

In practice the current scheme probably works OK, though I
wouldn't know due to lack of resources here :P