On Fri, Jul 23, 2010 at 11:40:58AM -0400, Jeff Moyer wrote: > > You are right, and we have to consider thinly provisioned luns, as well. > The only case I can think of where it makes sense to issue those > discards immediately is if you are running tight on allocated space in > your thinly provisioned lun. Aside from that, I'm not sure why you > would want to send those commands down with every journal commit, > instead of batched daily, for example. But, I can certainly understand > wanting to allow this flexibility. The two reasons I could imagine is to give more flexibility to the wear leveling algorithms (depending on how often you are turning over files --- i.e., deleting blocks and then reusing them), and to minimize latency (it might be nicer for the system to send down the deleted blocks on a continuing basis rather than to send them down all at once). The other issue is that by sending TRIM commands for all free extents, even those that haven't been recently been released, the flash translation layer needs to look up a large number of blocks in its translation table to see if it needs to update it. This can end up burning CPU unnecessarily, especially for those flash devices (such as FusionIO, for example) manage their FTL using the host CPU. So this is one of the reasons why I want to leave some flexibility here; BTW, for some systems, it may make sense for the FITRIM ioctl to throttle the rate at which it locks block groups and sends down TRIM requests so it doesn't end up causing performance hiccups for live applications while the FITRIM ioctl is running. - Ted -- 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
