linux-kernel - Re: [PATCH v4 44/68] cachefiles: Implement key to filename encoding

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Message-ID: <3149374.1641554392@warthog.procyon.org.uk>
Date:   Fri, 07 Jan 2022 11:19:52 +0000
From:   David Howells <dhowells@...hat.com>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     dhowells@...hat.com, linux-cachefs@...hat.com,
        Trond Myklebust <trondmy@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Matthew Wilcox <willy@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Omar Sandoval <osandov@...ndov.com>,
        JeffleXu <jefflexu@...ux.alibaba.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-afs@...ts.infradead.org, linux-nfs@...r.kernel.org,
        linux-cifs@...r.kernel.org, ceph-devel@...r.kernel.org,
        v9fs-developer@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 44/68] cachefiles: Implement key to filename encoding

Jeff Layton <jlayton@...nel.org> wrote:

> Since most cookies are fairly small, is there any real benefit to
> optimizing for length here? How much inflation are we talking about?

Taking AFS as an example, a vnode is represented at the file level by two
numbers: a 32-bit or 96-bit file ID and a 32-bit uniquifier.  If it's a 96-bit
file ID, a lot of the time, the upper 64-bits will be zero, so we're talking
something like:

	S421d4,1f07f34,,

instead of:

	S000421d401f07f340000000000000000

or:

	E0AAQh1AHwfzQAAAAAAAAAAA==

The first makes for a more readable name in the cache.  The real fun is with
NFS, where the name can be very long.  For one that's just 5 words in length:

	T81010001,1,20153e2,,a906194b

instead of:

	T8101000100000001020153e200000000a906194b

or:

	E0gQEAAQAAAAECAVPiAAAAAKkGGUs=

(The letter on the front represents the encoding scheme; in the base64 encoding
the second digit indicates the amount of padding).

I don't know how much difference it makes to the backing filesystem's
directory packing - and it may depend on the particular filesystem.

David