Messages in this thread | | | Date | Tue, 15 Apr 2003 18:28:24 +1000 | From | Nick Piggin <> | Subject | Re: Benefits from computing physical IDE disk geometry? |
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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
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