linux-kernel - Re: If not readdir() then what?

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Date:	Mon, 16 Apr 2007 06:39:50 -0400
From:	Theodore Tso <tytso@....edu>
To:	Neil Brown <neilb@...e.de>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Jörn Engel <joern@...ybastard.org>,
	Christoph Hellwig <hch@...radead.org>,
	Ulrich Drepper <drepper@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: If not readdir() then what?

On Mon, Apr 16, 2007 at 03:47:21PM +1000, Neil Brown wrote:
> "my guess", "pretty much" really bother me.
> 
> It sounds like "The largest anyone has asked for is 128bits, or let's
> double it and hope that is enough until the next protocol revision".
> Which was probably reasonable when NFSv2 was being developed and maybe
> even when v3 was developed, but I kind of hoped we were beyond that.
> 
> If a filesystem wanted to order filenames lexically, it really needs
> 256 *bytes*.  And it is fairly silly having a cookie that big.
> 
> I still thinking that 
>    filename + 64bits
> is required and plenty (aka necessary and sufficient).  

Sure, but if you're going to include the filename in the cookie, on
the client side you now have to store a variable-length state, which
probably means you'll need to allocate and free memory each time; and
if the filename is 256 characters, you'll have to send that back in
the next readdir() request. 

If we could get filename + 64bits, sure, that would be great.  I was
just assuming we couldn't get it --- and if we can't get it, 256 bits
is two SHA-1 hashes.  So that's one hash for the filename, and 128
bits for a filesystem's internal hash collision.  There might be other
ways that the space could be divided up, which might be somewhat
wasteful of space --- say you need a host identifier for a clustered
filesystem, although arguably adding a host might be infrequent enough
that you just use the cookie verifier hammer and force the client to
get a new set of readdir cookies.  :-)

> I wouldn't argue against 128bits (64 for a search key and 64 to
> guarantee uniqueness) but I really think 256 excessive with no value.
> We we still need the last-filename in the READDIR key.

I wouldn't complain too much about 128 bits, but if we're going to go
fixed size, I can imagine filesystems where that might not be enough.
And the differecen between 16 and 32 bytes isn't that great.  But I
could easily live with either filename + 64bits, or 128 bits.

						- Ted
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