Subject: Re: Question regarding to store file system metadata in database
From: Ming Zhang <mingz@ele.uri.edu>
Reply-To: mingz@ele.uri.edu
To: Xin Zhao <uszhaoxin@gmail.com>
Cc: mikado4vn@gmail.com, linux-kernel <linux-kernel@vger.kernel.org>,
       linux-fsdevel@vger.kernel.org
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	 <4ae3c140603190948s4fcd135er370a15003a0143a8@mail.gmail.com>
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Date: Sun, 19 Mar 2006 12:58:40 -0500
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database can reside on a raw block device.

but 300 metadata iops is not that fast. ;)

ming

On Sun, 2006-03-19 at 12:48 -0500, Xin Zhao wrote:
> well, the database could reside on another file system. So the
> database based file system could be a secondary file system but
> provide more features and  better performance. I am not saying that
> database-based file system must be the only filesystem on the system.
> 
> On 3/19/06, Mikado <mikado4vn@gmail.com> wrote:
> > -----BEGIN PGP SIGNED MESSAGE-----
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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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