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Re: [RFC] Union mount readdir support in glibc
- From: Jan Blunck <jblunck at suse dot de>
- To: Ulrich Drepper <drepper at redhat dot com>
- Cc: bharata at linux dot vnet dot ibm dot com, libc-alpha at sourceware dot org, Erez Zadok <ezk at cs dot sunysb dot edu>, linux-kernel at vger dot kernel dot org, linux-fsdevel at vger dot kernel dot org, viro at zeniv dot linux dot org dot uk, Christoph Hellwig <hch at lst dot de>, Mingming Cao <cmm at us dot ibm dot com>, Dave Hansen <haveblue at us dot ibm dot com>
- Date: Fri, 14 Mar 2008 16:07:12 +0100
- Subject: Re: [RFC] Union mount readdir support in glibc
- References: <20080311055527.GA7256@in.ibm.com> <47D9F6CC.6010009@redhat.com>
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.
Cheers,
Jan