On Thu, Oct 21, 2010 at 04:23:02PM +0200, Lukas Czerner wrote: > Since there are some slow SSD's out there and big thinly provisioned > storages on which it takes quite long to issue discard through whole > device, it would be nice to provide user the information about what is > going on and how long it will take (approximately). > > Signed-off-by: Lukas Czerner <lczerner@xxxxxxxxxx> Hi Lukas, I've looked at this patch, and one thing that disturbs me about it --- you are discarding the first percentage of the disk five percent times for no good reason just to get the timing, before then executing the discard for the entire disk. There are a couple of problems with this: *) For smart/competently implemented SSD's, discarding the same part of the disk five times might lead to a misleading timing --- the smart device could easily determine that the first 1% is already not in use after the first discard, and the subsequent 4 discards could be discard as no-ops. *) Mark Lord has claimed that there exists a large number of incomptently implemented SSD's out there, that may actually be executing a flash erase of the discarded region. If true, executing an extra flash erase on 1% of the disk for no good reason five times might not be the best thing to do for the longetivity of the device. I was tempted to fix this up myself, but since I'm trying to get better at delegating work to others, may I suggest the following changes? 1) Implement block device ioctl's for the kernel that export the discard_granularity, discard_alignment, and max_discard_sectors. 2) Change mke2fs so that the discard is done in a separate function. Said function should attempt to fetch the discard_granularity, discard_alignment, and max_discard_sectors. 3) This new function in mke2fs should start by discarding approximately 1% of device at a time, respecting discard_granularity and discard_alignment. If the time to discard 1% of the device is less than a second, then it should double the amount that it discards at a time. If the time to discard takes longer than 4 seconds, it should reduce the amount that it discards by half (again, always respecting discard_granularity and discard_alignment). The function can display the amount of time elapsed and the estimated amount of time remaining after each chunk of the device that it discards, assuming it can use ^M to redraw the progress report (which of course should be suppressed if the -q option is specified on the command line). This design doesn't "waste" any discards, which is both faster and reduces wear on badly designed SSD's. It also continuously updates the user with the amount of time it takes to complete the discard process. It also will respect the discard_granularity and discard_alignment restrictions; and of course, it allows the user to interrupt the discard, without needing a special kernel patch. Does this make sense to you? - 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
