Re: How capacious and well-indexed are ext4, xfs and btrfs directories?

Avi Kivity
2021-05-19T08:00:14Z

On 18/05/2021 02.22, Dave Chinner wrote:
>
>> 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.
> Note that if you use a single directory, you are effectively single
> threading modifications to your file index. You still need to use
> fanout directories if you want concurrency during modification for
> the cachefiles index, but that's a different design criteria
> compared to directory capacity and modification/lookup scalability.

Something that hit us with single-large-directory and XFS is that XFS
will allocate all files in a directory using the same allocation group.
If your entire filesystem is just for that one directory, then that
allocation group will be contended. We saw spurious ENOSPC when that
happened, though that may have related to bad O_DIRECT management by us.

We ended up creating files in a temporary directory and moving them to
the main directory, since for us the directory layout was mandated by
compatibility concerns.

We are now happy with XFS large-directory management, but are nowhere
close to a million files.