Re: Question regarding to store file system metadata in database

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

OK. Sorry for causing so much confusion here. I have to clarify
several things before go further on the discussion.

First, my experiment that resulted in 300 insertions/sec was set up as follows:
1. the testing code is written in python
2. I first creates a file list using "find /testdir -name "*" -print 
> filelist", and record current time after the filelist is created.
3. Then, I started a loop to read file pathnames line by line, for
each line, I do stat to get inode number, then I created a record and
insert it into database
4. after all records are inserted, I recorded current time again and
computed the elapsed time used to insert all records

>From this setting, we can see this experiment is not very fair for
database, because the time used to read filelist and do stat() are
also counted database insertion time. As noted before, I did that
experiment just to get some sense how slow a database could be. If I
remove the file read and stat() cost, I will expect to see an
improvement of insertion speed. I will redo the experiment and report
the result. Still, 300/sec might be good enough to handle most
scenarios. Yes. this might not be good enough to handle a busy web
server, while I still doubt a web server need to open so many files
per second. The frequently accessed files like small images are
commonly cached instead of requiring to access file system every time.

Second, I might want to give the background on which we are
considering the possibility of storing metadata in database. We are
currently developing a file system that allows multiple virtual
machines to share base software environment. With our current design,
a new VM can be deployed in several seconds by inheriting the file
system of an existing VM. If a VM is to modify a shared file, the file
system will do copy-on-write to gernerate a private copy for this VM.
Thus, there could be multiple physical copies for a virtual pathname.
Even more complicated, a physical copy could be shared by arbitrary
subset of VMs.  Now let's consider how to support this using regular
file system. You  can treat VMs as clients or users of a standard
linux.  Consider the following scenario: VM2 inherit VM1's file
system. The physical copy for virtual file F is F.1. Then, it modified
file F and get its private copy F.2. Now VM3 inherit VM2's file
system. The inherit graph is as follow:
VM1-->VM2-->VM3

Now VM3 wants to access virtual file F. It has to determine the right 
physical copy. The right answer is F.2. But in the file system, we
have F.1 and F.2. So some mapping mechanism must be devised. No matter
how we manipulate the pathname of physical copies, several disk
accesses seem to be required for a mapping operation. That is the
reason we are considering database to store metadata.

We do know many file systems already use db like technique to index
metadata. For example B tree used by ReiserFS and HTree used by Ext3.
But they do not provide the feature we need. This at least exposes one
fundamental limit: they do not support easy extension on metadata. So
at least some extension must be made to make the mapping efficient. So
we thought "since they are using db like technique, why not simply use
DB? " At least a DB makes it simple to extend metadata of a file
system. For example, in our case, we might also want to add hash value
of file content into a file's metadata. This allows us to merge
several files with identical contents into one for disk space saving,
which is important in our scenario since we assume that many VMs uses
identical software environment.

Also, I am not proposing to use db to store all metadata. As mentioned
before, currently I am just considering to store the pathname-inode
mapping. Other attributes like atime, ctime are stored using standard
way. So this is essentially a layer above standard FS. Because only
open () syscall needs to access metadata with the communication across
kernel boundary, I am expecting a moderate performance impact. But I
am not sure about this. Someone has any experience on that?

Any further comments?

Xin


On 3/20/06, Theodore Ts'o <tytso@mit.edu> wrote:
> On Sun, Mar 19, 2006 at 07:47:23PM +0000, Al Viro wrote:
> > As for "more efficient"...  300 lookups per second is less than an
> > improvement.  It's orders of magnitude worse than e.g. ext2; I don't
> > know in which world that would be considered more efficient, but I
> > certainly glad that I don't live there.
>
> There are two problems... well, more, but in the performance domain,
> at least two issues that stick out like a sore thumb.
>
> The first is throughput, and as Al and others have already pointed out
> 300 metadata operations per second is defintely nothing to write home
> about.  The second is latency; how much *time* does it take to perform
> an individual operations, especially if you have to do an upcall from
> the kernel to a userspace database process, the user space process
> then has to dick around its own general purpose,
> non-optimized-for-a-filesystem data structures, possibly make syscalls
> back down into the kernel only to have the data blocks pushed back up
> into userspace, and then finally return the result of the "stat"
> system call back to the kernel so the kernel can ship it off to the
> original process that called stat(2).
>
> Even in WinFS, dropped from Microsoft Longwait, it really wasn't using
> the database to store all metadata.  A better way of thinking about it
> is a forcible bundling of a Microsoft's database product (European
> regulators take note) with the OS; all of the low-level filesystem
> operations are still being done the traditional way, and it's only
> high level indexing operation which are being done in userspace (and
> only in userspace).  It would be like taking the taking the locate(1)
> userspace program and claiming it was part of the filesystem; it's
> more about packaging than anything else.
>
>                                                 - Ted
>
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