[TUHS] Musings on PDP-7 Unix

Hellwig Geisse hellwig.geisse at mni.thm.de
Mon Dec 26 08:57:14 AEST 2016


On So, 2016-12-25 at 18:39 +1000, Warren Toomey wrote:
> Filesystem
> ----------
> The implementation of the filesystem is, in some places, quite
> inefficient.
> The free block list is implemented as follows. In each block, there
> are
> 10 free block numbers then a pointer to the next part of the free
> list.
> However, each block can hold 64 block numbers, so why are only 10
> free
> block numbers stored in each block? By using the whole of a block to
> store
> free block numbers, there would actually be more free blocks to use!

Possibly there is a cache for free block numbers
located in the superblock? This is at least in V7
the reason for the limited number of free block
numbers contained in free blocks: if the cache
is to be refilled, the contents of a whole free
block must go into the cache, and this of course
cannot span a whole block, because its size is
only a fraction of the superblock's size.

Hellwig


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