Francois Barre [off-list ref] wrote:

2006/1/18, Mario 'BitKoenig' Holbe [off-list ref]:

quoted

Mario 'BitKoenig' Holbe [off-list ref] wrote:
Perhaps using some big (virtual) chunk size could do the trick? What

Stop me if I'm wrong, but this is called... huge readahead. Instead of
reading 32k on drive0 then 32k on drive1, you read continuous 512k
from drive0 (16*32k) and 512k from drive1, resulting in a 1M read.
Maybe for a single 4k page...

Yes, this would be the consequence.
However, this would probably not be a big issue, since a) the current
default read-ahead for RAID1 is 1024 (in 512-byte sectors) anyways.
Furthermore, in the hardware-RAID sector b) at least the 3ware support
recommends huge read-aheads for speeding up their RAID1s, too...
afaik they recommend:
vm.{min,max}-readahead=512
blockdev --setra 6144 /dev/...
which is far more than 1M. I don't know why they do so but I could
imagine they also use some strategy similar to the one I suggested.


regards
   Mario
-- 
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