Re: Notes on support for multiple devices for a single filesystem

Chris Mason
2008-12-17T21:41:58Z

On Wed, 2008-12-17 at 14:24 -0700, Andreas Dilger wrote:

> I can't speak for btrfs, but I don't think multiple device access from
> the filesystem is a "layering violation" as some people comment. It
> is
> just a different type of layering. With ZFS there is a distinct layer
> that is handling the allocation, redundancy, and transactions (SPA,
> DMU)
> that is exporting an object interface, and the filesystem (ZPL, or
> future
> versions of Lustre) is built on top of that object interface.

Clean interfaces aren't really my best talent, but btrfs also layers
this out. logical->physical mappings happen in a centralized function,
and all of the on disk structures use logical block numbers.

The only exception to that rule is the superblock offsets on the device.

-chris