Re: [PLUG] large_dir support on ext4

[Walt Mankowski via plug <plug@lists.phillylinux.org>]

On Fri, Sep 13, 2019 at 09:00:43AM -0400, Rich Freeman via plug wrote:
> On Fri, Sep 13, 2019 at 8:42 AM Walt Mankowski via plug
> <plug@lists.phillylinux.org> wrote:
> >
> > I used tune2fs to add large_dir support to the filesystem in hopes of
> > getting around this problem. It returned very quickly, which makes me
> > suspect it will only be applied to new directories, not existing
> > ones. I'm asking here on the off chance anyone might happen to know
> > for sure.
> >
> 
> I don't know the answer to that question, but you might want to
> consider also enabling dir_index.

dir_index is already enabled.

> Now, if you don't actually do name lookups but do something like using
> readdir to iterate through all the inodes in a directory then it won't
> provide benefit.  Anything that actually uses a filename to open a
> file directly would benefit, however, as otherwise the kernel
> basically has to scan the entire directory to find a file which I'm
> guessing is O(N).  I don't know enough about ext4 to know if the
> metadata can get fragmented, but I'm guessing it can, and thus that
> cost would apply to any seeks needed to read the entire directory
> listing.

I don't have any idea what rdiff-backup is doing, but just running

  ls | wc

on this directory took a good 10-15 minutes.

> I suspect that it doesn't work retroactively, so you will want to
> enable this before creating all your files.

Not really an option at this point, seeing as how it's an existing backup.

Walt

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