adventures in booting

[Søren Sandmann Pedersen]

Colin Walters wrote:

>>>I'm almost positive it requires kernel changes to do this the right way;
>>>one naive idea is to have a userspace daemon, capturing what blocks are
>>>read when (kernel tells this daemon using the kernel events layer). This
>>>would run in the first three minutes on each and every boot. When the
>>>system is idle (and only when running on AC power!) another daemon
>>>rearranges blocks on the disk. What blocks to rearrange could be the
>>>result of a computation involving several three-minute result sets.
>>>      
>>>
>>Rearranging sounds complex and dangerous, since it requires deep
>>integration with the filesystem.  The online resizing took quite a long
>>time to appear and that is conceptually much simpler.  Why not do it on
>>the block device layer (without knowledge of the filesystem) and just
>>copy those blocks to a reserved area of the block device?  Disks are
>>big, duplicating say 100MB for this purpose wouldn't be bad. 
>>    
>>
Doing the rearranging at the block level has several advantages:

   - We don't need to have this thread again (and don't need to apply
     another hack) when people realizes that OpenOffice *also* needs
     disk rearranging to start faster.

   - It is a general speedup across the system

   - If the disk blocks are generally arranged in such a way that
     blocks accessed together are close together, then readahead
     in the kernel becomes a matter of just reading further ahead
     *on the disk* instead of as now reading further ahead in the file.

   - when a set of blocks are read in, the VM system knows that
     those blocks are likely to be needed soon, so it can consider
     it a bad idea to throw those pages away.


Søren