[Nottingham] LVM and load balancing and ReiserFS

[Charles Samuels]

Martin wrote, on Saturday 2007 October 27:
> Does ReiserFS optimise to use HDD cylinders to minimise head movements?
> If so, will ReiserFS still pick that up from LVM?...

No, it doesn't, and it can't, because there's no way of knowing if "cylinder 
n" is next to "cylinder n+1" (and so forth). What the hard drive claims is 
its geometry is going to be completely fake, for a number of reasons 
including:

- They only manufacture one size of disk drive these days, 500GB or more. The 
ones that have too many bad sectors get remapped and sold as as high as they 
can make it (400, 300, 250, whatever).

- They map certain locations to be on the outer edge of the platters so that 
Windows boots faster (wild guess: they don't actually do this anymore)

What probably does happen is that the IO layer on Linux is asked to read a 
bunch of sectors, and that list goes to the harddrive whose firmware does 
what it can to read them all as fast as possible, in whatever order.


Out of curiosity, why is LVM so popular? Software RAID on linux (md) is really 
easy to use and incredibly reliable as well, and it also can do striping and 
mirroring.

Charles
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