Messages in this thread |  | | Date | Mon, 26 Nov 2001 17:15:13 -0800 | From | "H. Peter Anvin" <> | Subject | Re: Journaling pointless with today's hard disks? |
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Andrew Morton wrote:
> "H. Peter Anvin" wrote: > >>Followup to: <9tumf0$dvr$1@cesium.transmeta.com> >>By author: "H. Peter Anvin" <hpa@zytor.com> >>In newsgroup: linux.dev.kernel >> >>>Indeed; having explicit write barriers would be a very useful feature, >>>but the drives MUST default to strict ordering unless reordering (with >>>write barriers) have been enabled explicitly by the OS. >>> >>> >>On the subject of write barriers... such a setup probably should have >>a serial number field for each write barrier command, and a "WAIT FOR >>WRITE BARRIER NUMBER #" command -- which will wait until all writes >>preceeding the specified write barrier has been committed to stable >>storage. It might also be worthwhile to have the equivalent >>nonblocking operation -- QUERY LAST WRITE BARRIER COMMITTED. >> >> > > For ext3 at least, all that is needed is a barrier which says > "don't reorder writes across here". Asynchronous behaviour > beyond that is OK - the disk is free to queue multiple transactions > internally as long as the barriers are observed. If the power > goes out we'll just recover up to and including the last-written > commit block. >
Waiting for write barriers to clear is key to implementing fsync() efficiently and correctly.
-hpa
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