Re: Question regarding to store file system metadata in database

["Xin Zhao" <uszhaoxin@gmail.com>]

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:
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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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