On Aug 26, 2009 09:14 -0400, Theodore Ts'o wrote: > On Wed, Aug 26, 2009 at 03:50:35AM -0600, Andreas Dilger wrote: > > I'm not sure I understand about the "n.b." case. If the filesystem > > is running with !async_commit,barrier=0,"hdparm -W 0" (which is basically > > ext3 with write cache off), it should still have the jbd code doing > > an explicit wait for the data blocks (which should be guaranteed to > > make it to disk, wcache being off) before even submitting the commit > > block to the elevator? It doesn't matter what order the transaction > > blocks are written to disk, so long as the commit block is last. > > Gack, sorry, I screwed that up. What I should have written is this: > > The safe configurations people could try benchmarking: > > !async_commit,barrier=1,"hdparm -W 1" (currently the default) > !async_commit,barrier=0,"hdparm -W 0" > async_commit,barrier=1,"hdparm -W 1" > > and the unsafe case in the nb should have been <async_commit, > barrier=0, "hdparm -W 0">, since without the barrier, async_commit > writes the commit block at the same time as the rest of the journal > (data, metadata, and revoke) blocks, and so there is the chance the > commit block could get reordered in front of the other journal blocks. I'm still missing something. With async_commit enabled, it doesn't matter if the commit block is reordered, since the transaction checksum will verify if all of the data + commit block are written for that transaction, in case of a crash. That is the whole point of async_commit. If the commit block is on disk, but there are some transaction blocks missing the checksum will (except in very rare coincidences) fail and the transaction is skipped. With "hdparm -W 0" we are guaranteed to only have a single uncommitted transaction, except in the case of journal corruption (i.e. disk error or software bug). I can imagine with "async_commit,barrier=0,"hdparm -W 1" that having multiple transactions begin checkpointing before they are fully committed, which means the filesystem is modified in a non-recoverable way. Cheers, Andreas -- Andreas Dilger Sr. Staff Engineer, Lustre Group Sun Microsystems of Canada, Inc. -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
