Re: Question regarding to store file system metadata in database

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

OK. Now I have more experimental results.

After excluding the cost of reading file list and do stat(), the
insertion rate becomes 587/sec, instead of 300/sec. The query rate is
2137/sec. I am runing mysql 4.1.11. FC4, 2.8G CPU and 1G mem.

2137/sec seems to be good enough to handle pathname to inode
resolving.  Anyone has some statistics how many file open in a busy
file system?

Xin


On 3/20/06, Xin Zhao <uszhaoxin@gmail.com> wrote:
> 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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