[Linux-cachefs] How capacious and well-indexed are ext4, xfs and btrfs directories?

[Andreas Dilger]

On May 17, 2021, at 9:06 AM, David Howells <dhowells at redhat.com> wrote:
> With filesystems like ext4, xfs and btrfs, what are the limits on directory
> capacity, and how well are they indexed?
> 
> The reason I ask is that inside of cachefiles, I insert fanout directories
> inside index directories to divide up the space for ext2 to cope with the
> limits on directory sizes and that it did linear searches (IIRC).
> 
> For some applications, I need to be able to cache over 1M entries (render
> farm) and even a kernel tree has over 100k.
> 
> What I'd like to do is remove the fanout directories, so that for each logical
> "volume"[*] I have a single directory with all the files in it.  But that
> means sticking massive amounts of entries into a single directory and hoping
> it (a) isn't too slow and (b) doesn't hit the capacity limit.

Ext4 can comfortably handle ~12M entries in a single directory, if the
filenames are not too long (e.g. 32 bytes or so).  With the "large_dir"
feature (since 4.13, but not enabled by default) a single directory can
hold around 4B entries, basically all the inodes of a filesystem.

There are performance knees as the index grows to a new level (~50k, 10M,
depending on filename length)

As described elsewhere in the thread, allowing concurrent create and unlink
in a directory (rename probably not needed) would be invaluable for scaling
multi-threaded workloads.  Neil Brown posted a prototype patch to add this
to the VFS for NFS:

https://lore.kernel.org/lustre-devel/8736rsbdx1.fsf@notabene.neil.brown.name/

Maybe it's time to restart that discussion?

Cheers, Andreas





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