Re: Question regarding to store file system metadata in database

[Mikado]

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Where is that database located, on other filesystem or on database-based
filesystem?

Xin Zhao wrote:
> I was wondering why only few file system uses database to store file
> system metadata. Here, metadata primarily refers to directory entries.
> For example, one can setup a database to store file pathname, its
> inode number, and some extended attribution. File pathname can be used
> as primary key. As such, we can achieve pathname to inode mapping as
> well as many other features such as fast search and extended file
> attribute management. In contrast, storing file system entries in
> directory files may result in slow dentry search. I guess that's why
> ReiserFS and some other file systems proposed to use B+ tree like
> strucutre to manage file entries. But why not simple use database to
> provide the same feature? DB has been heavily optimized to provide
> fast search and should be good at managing metadata.
> 
>  I guess one concern about this idea is  performance impact caused by
> database system. I ran a test on a mysql database: I inserted about
> 1.2 million such kind of records into an initially empty mysql
> database. Average insertion rate is about 300 entries per second,
> which is fast enough to handle normal file system burden, I think.  I
> haven't try the query speed, but I believe it should be fast enough
> too (maybe I am wrong, if so, please point that out.).
> 
> Then I am a little curious why only few people use database to store
> file system metadata, although I know WinFS plans to use database to
> manage metadata. I guess one reason is that it is difficult for kernel
> based file system driver to access database. But this could be
> addressed by using efficient kernel/user communication mechanism.
> Another reason could be the worry about database system. If database
> system crashes, file system will stop functioning too. However, the
> feature needed by file system is really a small part of database
> system, A reduced database system should be sufficient to provide this
> feature and be stable enough to support a file system.
> 
> Can someone point out more issues that could become obstables to using
> database to manage metadata for a file system?
> 
> Many thanks!
> Xin
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