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 means any time the file has no open file descriptors it could "vanish" if the underlying filesystem wants to use the space it occupies for something else. A file could have a "priority" value which specifies how important it is, and therefore how likely it is to be erased if a new block needs to be allocated. The block allocator would use this information, together with the physical layout of the data to decide where to place new data to avoid fragmentation while retaining possibly useful data for future use. Let me know your ideas on this - at the moment it's only an idea, but I'm interested to know if a) it would be possible to implement it into a complex filesystem like btrfs, and b) if it would prove useful if implemented. Thanks Oliver. PS. I realise this could be implemented with a user space daemon which polls available disk space and deletes caches when disk space gets low (as windows does with shadow copies), but that hardly seems ideal, since it can't intelligently choose which caches to delete to reduce fragmentation, and large sudden disk allocations will fail. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
