On Tue, 2013-01-29 at 15:04 -0800, Andy Grover wrote: > On 01/29/2013 11:03 AM, Nicholas A. Bellinger wrote: > > > So enabling emulate_write_cache=1 in the case when the underlying device > > has not enabled it is incorrect. > > > > I'd like to enable this bit when we know the underlying device has WCE=1 > > set, but currently there is not a way to determine this (generically) > > from struct block_device. > > > > So NACK for applying this until there is a method to determine what the > > hardware below is doing. > > This should be possible from userspace though. I'm planning on looking > up underlying scsi device(s?) using libblkid, and then query the sense > data using libsgutils when adding a block backstore in targetcli. > Querying the mode pages from userspace would work for the SCSI backstore case, but certainly not for raw block devices. I'd still like to see this exposed to the block layer somehow, so that the setting can be automatically determined by TCM once it's known if the underlying HW has enabled write caching. > It's kind of a hassle, but isn't it a huge performance win if we can > enable this? > Most certainly, but the danger is reporting WCE=1 (by default in all cases) from TCM to the initiator when the underlying drives do not have caching enabled. Note that every spinning media device that I've ever seen disables WCE by default from the factory. --nab -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
