Re: Question regarding to store file system metadata in database

[Pavel Machek <pavel@suse.cz>]

Hi!

> 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.

Hardlinks? ext3cow (google it)?
								Pavel

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