[lkml]   [2008]   [Mar]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [RFC] Union mount readdir support in glibc
On Thu, Mar 13, Ulrich Drepper wrote:

> There is very little overhead. Since we copy using getdents multiple
> records it is more efficient than implementing readdir in the kernel.
> This is how efficient normal directory operations must remain. The only
> slight inefficiency is that we have to copy the entries after getdents()
> because the d_type field is not in the place we expect it at userlevel.
> For this a new interface could help.

BTW, Since some filesystem always give DT_UNKNOWN an additional stat is
necessary to implement whiteout filtering. I don't want to do that in
kernel-space if possible ...

> Regarding questions you have: if a directory currently is read and file
> are added or removed, all bets are off.
> re seeking: you have to support seeking. There is no way around it.
> Once again, if any file has been added/removed, all bets are off. So,
> why not provide a cookie similar to what is done today? I think it is
> not acceptable to require caching the entire directory content at
> userlevel. It's bad enough if we have to store the file names for
> duplicate elimination.

Which basically means tracking of the "space" between dirents and maintaining
the relative order of entries. Which is a pain. I already tried to solve this
problem for tmpfs before and it needs a hugh amount of kernel memory for open
directories. In the end I only know of one situation where it is used: very old
glibc when running 32bit applications on 64bit kernel.


 \ /
  Last update: 2008-03-14 16:09    [W:0.076 / U:1.480 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site