On Fri, Apr 29, 2011 at 11:30:47AM +0200, Lukas Czerner wrote: > 1. We need to honor all the "discard limits" so the discard bios does > not actually fail. > 2. If the device is composed of various other devices, we should return > -EOPNOTSUPP is none of the devices support discard. > 3. We should succeed, if at least one of the devices support discard and > it does not fail for any reason. > 4. We should not advertise discard_zeroes_data if any of the devices > does not zero data or does not support discard. > I am not sure how "hard" is to assure those conditions in DM. If those > conditions are met, we can rely on consistent information in the layers > above. Remember that -EOPNOTSUPP return applies only to that one *specific* discard. It does not tell you for sure whether or not another future discard to the device will succeed. (It's a property of offset - if there are several devices underneath - and of time - if the device or one below it gets reconfigured.) The core issue here is whether a filesytem should decide that the receipt of a single -EOPNOTSUPP is a reason never to send any more, whether a more sophisticated algorithm should be used (considering the proportion/offsets of them over given periods of time and retrying later), or whether more comprehensive information about the discard capabilities of the device should be presented - and whether this should be handled automatically or whether it should be under userspace control (i.e. the sysadmin can instruct the filesystem what to do). dm's role is simply to handle a discard if it can, and report back if it couldn't. Additionally it could provide more comprehensive information about the discard capabilities of the device, but my sense is that most consider this unnecessary as normally dm devices will have a coherent behaviour throughout. Alasdair -- 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
