On Tue, 08 Jun 2010 14:23:04 -0400 Iordan Iordanov <iordan@xxxxxxxxxxxxxxx> wrote: > Hi, > > We are designing a rather involved file server with an ext3 formatted > stripe (raid0) sitting on top of raid10 devices. Each raid10 device sits > on top of 3 iscsi targets, and has layout n3 (so it is effectively a 3 > way mirror). We chose raid10 over raid1 due to an apparent read > performance benefit of raid10. > > We are trying to decide whether to upgrade to a kernel newer than > 2.6.34, where write barriers are ostensibly supported by all possible > raid types, because we are worried about ext3 corruption with no write > barrier support. > > However, we are also worried about whether write barriers really "make > sense" in a multi-disk environment and are wondering whether they will > actually make a difference in our setup. For argument's sake, let's > assume that our drives honor write cache flushes. > > Can somebody shed some light on how write barriers are implemented in > raid0 and raid10? Also, any critical comments on the validity of our > setup and/or assumptions is also welcome. Write barriers are handled as: - drain all outstanding requests and block new requests - send a zero-length barrier to each component device - send the data in the barrier request (if it wasn't zero length) - send anther zero-length barrier if there was data - allow new requests through. So you may well notice a slow-down i you enable barriers, but theoretically your data might be a bit safer. But that is what barriers have always meant for ext3, which is why they aren't enabled by default. NeilBrown -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
