On Mon, Nov 22, 2010 at 6:29 AM, Lukas Czerner <lczerner@xxxxxxxxxx> wrote: > It takes fstrim_range structure as an argument. fstrim_range is definec in > the include/linux/fs.h. > > After the FITRIM is done, the number of actually discarded Bytes is stored > in fstrim_range.len to give the user better insight on how much storage > space has been really released for wear-leveling. > > Signed-off-by: Lukas Czerner <lczerner@xxxxxxxxxx> That's a misleading description in my opinion, from my understanding this is more accurate: === After the FITRIM is done, the number of bytes passed from the filesystem down the block stack to the device for potential discard is stored in fstrim_range.len. This number is a maximum discard amount from the storage device's perspective, because FITRIM called repeated will keep sending the same sectors for discard repeatedly. fstrim_range.len will report the same potential discard bytes each time, but only sectors which had been written to between the discards would actually be discarded by the storage device. Further, the kernel block layer reserves the right to adjust the discard ranges to fit raid stripe geometry, non-trim capable devices in a LVM setup, etc. These reductions would not be reflected in fstrim_range.len. As 2.6.37, the kernel block layer does not fully support discard and as such will simply ignore all discard requests sent to volumes created by device mapper or mdraid. This is done in a silent way, so these failures to discard are also not reflected in fstrim_range.len. Thus fstrim_range.len can give the user better insight on how much storage space has potentially been released for wear-leveling, but it needs to be one of only one criteria the userspace tools take into account when trying to optimize calls to FITRIM. === Obviously, I'd like to also see that also in API documentation for FITRIM. (And correct me if I'm wrong about device mapper / mdraid. I'd love to be wrong about that statement..) Greg -- 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
