Re: Partial allocation

Hi,

I presume that in the design of BTRFS, like most other filesystems, a
block on the underlying storage is either allocated (ie. to store
metadata or file data), or deallocated (possibly blank or containing
garbage left over, but the contents are irrelivant).

Does BTRFS have any system that could allow adding at a later point in
time a feature which would allow "weak" allocation of blocks, by which I
mean the block is allocated (ie. storing useful data), but if another
file needs to be written which has a higher priority and there are no
free blocks left, then the data will be replaced.

I could forsee uses for features like that as a cache - for example my
web browsing cache is not vital data, and as such doesn't need to use up
disk space, but it might as well use up any disk space that would
otherwise go unused.  The cache data can always be regenerated, so
loosing the data isn't a problem.

Other uses of the feature could be for persistant network caches (ie. to
store copies of remote files on the network can they can be accessed
faster locally), but again the cache data isn't critical to the
operation of the system, so could be stored in "weakly allocated"
blocks.  Further uses could be caches of compressed files (decompressed
versions of the same files are also saved in other blocks, and depending
on IO and CPU load either the compressed or decompressed version is
used).

>From a user-land perspective, these files could be created with a
special flag which specifies they are only "weakly allocated", which