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List:       reiserfs-devel
Subject:    Re: issues related to writing a file in reiser4
From:       Hans Reiser <reiser () namesys ! com>
Date:       2005-03-24 18:08:36
Message-ID: 42430224.8050708 () namesys ! com
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Pallavi Dalya wrote:

> hello,
>   By doing debugfs on reiser4 and studying it I understand that the
> disk block address of the root node of on-disk tree as well as the
> complete on-disk tree gets  changed due to the creation of extent pointer.
> For example:: suppose the root node is initally created at disk block
> 23. And the other nodes of the tree are say 24,25,26. Now suppose a
> extent gets created. This extent  has a length of 30 disk blocks.So
> now according to my observation the extent starts from disk block
> number 23 and extends till disk block number (23+30)53. and now the
> root node of the tree starts from block number 54..and the other nodes
> of the tree starts from 55,56,57 respectively.
> This means that whenever an extent is created the extents are stored
> and then the on-disk tree is stored to maintain locality of space.
> What is the advantage of such a policy???Isn't it shear overhead?

Sheer overhead compared to what?  I don't understand the question well.

>      Secondly when a file is created its entry has to be made in its
> respective directory i.e. in the directory entry item. The file is
> assigned the key which is the value that the field "next-oid" in the
> super block holds. But no matter when the file is created or what key
> it is assigned it is stored alphabetically in the tree. Even the stat
> data and the tail item of the respective file are inserted
> alphabetically. What is the use of alphabetical insertion? Is this
> useful in anyway during searching?

If directory entries and file bodies are stored in the same order, then
it creates a dramatic performance improvement for when accessing things
in readdir order.

>       Also if files are stored alphabetically and not according to the
> key order.Since the keys are generated sequentially but files are
> stored alphabetically the keys appear to be in random order on the
> disk. In this case how can the concept of delimiting keys be used???

I don't understand the question.  File body parts are stored by key. 
Keys include the filename as one component unless it is more than 15 (or
16, can't remember....) characters.

> regards,
> pallavi
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