Created attachment 363857 [details]
Preserve he order in which the files are used when writting the files lists
In the case of SSDs, it is not of much use to sort the files lists by block order. It is more important to readahead the files in the order they will be needed by the programs, so that readahead can be backgrounded.
As a first step, it is necessary to preserve the order in which the files are accessed. The attached (or to be attached, depending on whether bugzilla likes it or not) patch does exactly that.
Hmm, does it make sense for SSDs anyway?
Sorting is mainly because of seek time, which is the main gain from readahead.
For SSDs starting initscripts in parallel would make the most sense (like upstart does in newest Ubuntu).
Starting init scripts in parallel is helpful in all cases, on Debian we now use a dependencies-based rc which allows us to start scripts in "makefile" mode, i.e. as soon as their dependencies are satisfied. The boot time is reduced in all cases.
In the case of SSDs, using readahead as a backgrounded, low priority, preloading mechanism of the files still reduces the boot time. The trick in the case of SSDs is to preload in the order in which the files will be used. Ideally readahead will have preloaded the files needed by an application before the application requests it to the kernel.
In the case of HDDs the best performance is achieved by preloading by the blocks order with a normal io priority, in the foreground, without having any other application that may request disk access running at the same time.
 I do have plans to make readahead work this way, but some major code structure changes are needed. Hope I can count on you to make that happen.
This bug appears to have been reported against 'rawhide' during the Fedora 12 development cycle.
Changing version to '12'.
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