On Mon, 22 Nov 2010, Ted Ts'o wrote: > 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 > Hi Ted, this absolutely make sense to me. I like the idea way better than what I had done in my patch (actually I was probably lazy to do it right in the first place:)). So, I'll add this into my todo list and hopefully find some time to deal with it ASAP. Thanks for suggestions! -Lukas -- 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
