On Wed, 30 Mar 2011, Arnd Bergmann wrote: > On Monday 28 March 2011, Kyungmin Park wrote: > > So you will go through (blocks, bytes...) 0 -> 20 > > > > OOOO==O===OO===OOOOO==O===O===OOOOOOO=== > > ^ ^ > > 0 20 > > > > So, you will call discard on extents: > > > > 0-3 > > You'll skip 6 because is smaller than minlen > > 10-11 > > 15-19 > > > > instead of > > > > 0-3 > > 10-11 > > 15-19 > > 30-36 > > Sorry for joining the discussion late, but shouldn't you also pass > the alignment of the discards? > > FAT is typically used on cheap media that have very limited support > for garbage-collection, such as eMMC or SD cards. > > On most SDHC cards, you only ever want to issue discard on full erase > blocks (allocation units per spec), typically sized 4 MB. I was not aware of the fact that SD cards (etc..) does have garbage collection of some sort, or that they even have support discard, since I thought that we have only TRIM,UNAMP/WRITE_SAME comands for SATA or SCSI drives. Or is there some sort of kernel mechanism doing garbage collection such is this for the cheap media ? > > If you just pass the minimum length, the file system could end up > erasing a 4 MB section that spans two half erase blocks, or it > could span a few clusters of the following erase block, both of > which is not desirable from a performance point of view. Does those cards export such information correctly ? > > On other media, you have the same problem inside an erase block: > These might be able to discard parts of an erase block efficiently, > but normally not less than a flash page (typically 8 to 32 KB). Well I have tested several SSD's and thinly provisioned devices, but I have not seen any strange behaviour, other than it was terribly unefficient to do so. See my results here: http://people.redhat.com/lczerner/discard/test_discard.html the fact is that I have not tried discard size smaller than 4K, since this is the most usual block size for the filesystem. > > Again, you don't want to discard partial pages in this case, and > that is much more important than discarding a large number of pages > because it would result in an immediate copy-on-write operation. > > Further, when you erase some pages inside of an erase block, you > probably should not span multiple erase blocks but instead issue > separate requests for each set of pages in one erase block. Does it mean that we should not issue bigger discards that erase block ? That does not sound good given my test results. Or maybe I misunderstood your point ? > > Arnd > > Arnd > Thanks! -Lukas -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
