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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Date: Sun, 19 Mar 2006 13:26:27 -0500
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no. i have no such statistics. also people always want it to be faster,
so it is never enough.

from another point of view, if such fs is used by a mail server, large #
of file create/close/modify will be vital for it. 300/s is not enough
for a busy mail server of course.

database based file system will be useful for archiving. for heavy
online use? not sure.

also will a database based fs too be too complex while all benefits
brought by db can be brought by add-on utilities? find and grep do not
fit u bill?

ming

On Sun, 2006-03-19 at 13:11 -0500, Xin Zhao wrote:
> Do you have any statistics on how many metadata accesses are required
> for a heavy load file system?  I don't have on in hand, but
> intuitively I think 300 per second should be enough. If storing
> metadata in database will not hit the file system performance, plus
> database allows flexible file searching, the database-based file
> system might not be a bad idea. :)
> 
> Xin
> 
> On 3/19/06, Ming Zhang <mingz@ele.uri.edu> wrote:
> > 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-----
> > > > Hash: SHA1
> > > >
> > > > 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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> >

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