linux-ext4 - Re: [RFC] A proposal for adding case insensitive lookups to ext4

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Message-ID: <20161105000052.54dmfdp5sgiqq6wo@thunk.org>
Date:   Fri, 4 Nov 2016 20:00:52 -0400
From:   Theodore Ts'o <tytso@....edu>
To:     Jeremy Allison <jra@...gle.com>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Daniel Rosenberg <drosen@...gle.com>, guy@...ux.com
Subject: Re: [RFC] A proposal for adding case insensitive lookups to ext4

On Fri, Nov 04, 2016 at 04:28:05PM -0700, Jeremy Allison wrote:
> I don't suppose ext4 has a negative cache for lookups ? That would
> certainly help the linear search case on lookup miss.

The dcache caches negative results, so as long as the lookup miss is
for the same non-existing file name, that's not a problem.

The issue will be if someone is using Makefile with default rules,
say, and Makefile is checking for foo.y, foo.l, foo.C, etc. for each
object file, there could be a fairly large number of failed lookups
that might require O(n) lookups.  Eventually all of these will be
cached, yes, but it could be a large number of negative dentry caches.

However, I'm not sure I care; no self-respecting programmer should be
using a case-insensitive file system, so in practice, I'm not sure it
matters all that much....

					- Ted
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