Re: Proposal: B_ARRIER (addresses wapbl performance?)

[Joerg Sonnenberger <joerg%britannica.bec.de@localhost>]

On Tue, Dec 09, 2008 at 12:12:32PM -0800, Jason Thorpe wrote:
> You want your journal to remain self-consistent, otherwise you can't  
> trust it to replay it.

If each journal entry is checksummed and the it is ensured that old data
is not valid, you can write the journal async as long as the journal
hits the disk before the corresponding meta data. The minimal
constraints are:
(1) Newly allocated blocks are either in the journal or are written
before the corresponding journal entry. (*)
(2) Blocks in the journal are written after the journal entry and all
previous journal blocks have been written to stable storage.

(*) is currently not done. Depending on how to ensure that old data is
not valid anymore, zeroing of the journal could be used and have
similiar constraints.

The question is how this can be mapped to the physical constraints. For
ATA, it can be effectively only done using a flush after (1) and before
(2), but the latter doesn't have to be done after each journal entry,
just when flushing the journal content to disk.

How does this look for more intelligent devices?

Joerg