[fuse] Speeding up readdir()

[Nikolaus Rath <Nikolaus@rath.org>]

Hello,

I am trying to improve the performance of readdir() requests. I have a
client application that issues a lot of readdir() requests, and a FUSE
filesystem that makes extensive use of kernel caching for inode entry
attributes because retrieving the attributes from the backend is
relatively expensive.

Unfortunately, it seems to me that currently there is no way to avoid
having to retrieve the attributes from the backend for every entry that
is returned by readdir - on every call:

If I am using readdirplus, I have to include the full attributes even if
the kernel already has them cached.

If I disable readdirplus, I can return just the entry name and its inode
- but I believe because this doesn't result in a lookup count increase
of the inode, the kernel can't match this with the existing cached data
for the inode (is that correct?) and I'm getting a separate lookup()
request for each entry that I've returned.


I could implement a readdir cache in the filesystem, but that means I
have to take care of cache invalidation and I'm basically wasting
memory.

Is there a reason why readdirplus() couldn't return just the name and
inode, together with a special flag that tells the kernel to "just use
the attributes that are already cached"?


Best,
-Nikolaus

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