On Jan 23, 2006, at 08:52:51, Antonio Vargas wrote:
On 1/23/06, Kyle Moffett <[email protected]> wrote:
The way that I'm considering implementing this is by intentionally
fragmenting the allocation bitmap, catalog file, etc, such that
each 1/8 or so of the disk contains its own allocation bitmap
describing its contents, its own set of files or directories,
etc. The allocator would largely try to keep individual btree
fragments cohesive, such that one of the 1/8th divisions of the
disk would only have pertinent data for itself. The idea would be
that when trying to look up an allocation block, in the common
case you need only parse a much smaller subsection of the disk
structures.
this sounds exactly the same as ext2/ext3 allocation groups :)
Great! I'm trying to learn about filesystem design and
implementation, which is why I started writing my own hfsplus
filesystem (otherwise I would have just used the in-kernel one). Do
you have any recommended reading (either online or otherwise) for
someone trying to understand the kernel's VFS and blockdev
interfaces? I _think_ I understand the basics of buffer_head,
super_block, and have some idea of how to use aops, but it's tough
going trying to find out what functions to call to manage cached disk
blocks, or under what conditions the various VFS functions are
called. I'm trying to write up a "Linux Disk-Based Filesystem
Developers Guide" based on what I learn, but it's remarkably sparse
so far.
One big question I have: HFS/HFS+ have an "extents overflow" btree
that contains extents beyond the first 3 (for HFS) or 8 (for HFS+).
I would like to speculatively cache parts of that btree when the
files are accessed, but not if memory is short, and I would like to
allow the filesystem to free up parts of the btree under the same
circumstances. I have a preliminary understanding of how to trigger
the filesystem to read various blocks of metadata (using
buffer_heads) or file data for programs (by returning a block number
from the appropriate aops function), but how do I allocate data
structures as "easily reclaimable" and indicate to the kernel that it
can ask me to reclaim that memory?
Thanks for the help!
Cheers,
Kyle Moffett
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