[rfc] boot-time async readahead...

[Phillip Susi]

Scott James Remnant wrote:
> Sync is *always* better than no readahead at all.
> Parallel is *sometimes* better than no readahead, but in various cases
> is actually _worse_.
> When Parallel is not worse then no readahead, it is better than sync.
> 
> Since the out-of-the-box has to work for everyone, I chose sync.

You may be able to get the best of both worlds.  Take the readahead list 
sorted in order of access.  Split the list in half.  Sort each 
individual half by block address.  Now do a sync readahead of the first 
list, and async readahead of the second.  Set the io priority of the 
async readahead to a very high value so that should a startup process 
request a block that has not yet been read ahead, it will not cause a 
seek, but instead will just wait for readahead to get there in the right 
order.

Also it sure would be nice to defrag the disk so the files are placed in 
the order they are accessed.