linux-ext4 - Re: Htree concept

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Message-ID: <mj0250$tvl$1@ger.gmane.org>
Date:	Wed, 13 May 2015 19:37:36 +0200
From:	"U.Mutlu" <for-gmane@...luit.com>
To:	linux-ext4@...r.kernel.org
Subject: Re: Htree concept

U.Mutlu wrote on 05/13/2015 07:22 PM:
> Eric Sandeen wrote on 05/13/2015 06:29 PM:
>> On 5/13/15 10:37 AM, U.Mutlu wrote:
>>> Hi,
>>> I'm writing a toy-fs, and discover a major shortcoming
>>> (finding a given child (dir/file) as fast as possible),
>>> which other developers (ie. ext3/4) had encountered long ago too.
>>> They introduced HTree. The info on HTree on the web is scarce
>>> or I couldn't find the right texts/papers yet.
>>> I wonder how HTree works on a conceptual basis.
>>> Could a kind soul enligten me pls. TIA.
>>
>> Regarding htree details, did you look at:
>>
>> http://en.wikipedia.org/wiki/HTree
>>
>> which points to:
>>
>> http://ext2.sourceforge.net/2005-ols/paper-html/node3.html
>> and more specifically,
>> http://web.archive.org/web/20131203105316/http://www.linuxshowcase.org/2001/full_papers/phillips/phillips_html/index.html
>>
>>
>> ?
>
> Thanks, the wiki page and its refs I knew, but needed some more info.
>
> Ok, it is written that HTree uses 32bit (or 64?) hashes for keys.
> I wonder if it wouldn't be better if one instead would use that space
> (32/64 bit) for storing the first n chars of the key (ie. of the dir/file name)
> and keeping the directory entries in a sorted order on the disk,
> and then do a bsearch instead of doing sequential table lookup using HTree?
> I wonder what the "Tree"-part of HTree stand for in this context.
> Am I right in my assumption that HTree mainly means the hashing mechanism,
> but does not use any binary search mechanism for searching the key?

Addendum:
I think I slowly grasp how HTree works: it keeps a (rb/avl tree)
b*tree-db (I guess it stores it on disk) of the hashes (as keys).

In contrast to that here my idea: keep the hdr blocks (ie. where the
dir/file names are) always in a sorted order. Then a bsearch should be doable.
This would eliminate the need for any b*tree-db usage.

-- 
cu
Uenal


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