31 Dec
2021
31 Dec
'21
9:08 a.m.
On Thu, Dec 30, 2021 at 05:31:09PM -0500, Dan Cross wrote:
On Thu, Dec 30, 2021 at 11:41 AM Theodore Ts'o
<tytso(a)mit.edu> wrote:
I was responding to Bakul's suggestion that the original Unix
could/should have stored the cwd as a string, instead of a pointer to
a directory inode. If you stored the cwd as a string, then you could
interpret .. by chopping off the last file name component of the
string, and so now you could have hard links to directories and the
file system tree could then be a file system DAG.
I don't think Bakul's proposal would have worked --- although I
suppose if you disallowed directory renames, I guess it could. Noel's
objection above was a performance one, in that if you had to parse the
path every single time, you'd be much more dependent on the namei
cache --- and that PDP-11 didn't have the memory to support a
sufficiently large cache.
- Ted
On Wed, Dec 29, 2021 at 10:45:12PM -0500, Noel
Chiappa wrote:
The other problem with storing the path as a string is that if
higher-level directories get renamed, the path would become
invalidated. If you store the cwd as "/foo/bar/baz/quux", and someone
renames "/foo/bar" to "/foo/sadness" the cwd-stored-as-a-string
would
become invalidated.
Why? Presumably as you traversed the filesystem, you'd cache, (path
component, inode) pairs and keep a ref on the inode. For any given
file, including $CWD, you'd know it's pathname from the root as you
accessed it, but if it got renamed, it wouldn't matter because you'd
have cached a reference to the inode. From:
Bakul Shah
My guess is *not* storing a path instead of a ptr
to the inode was done
to save on memory.
More probably speed; those old disks were not fast, and on a PDP-11, disk
caches were so small that converting the path to the current directory to its
in memory inode could take a bunch of disk reads.