On 07/03/2018 16:57, Max Reitz wrote: >>>> (2) For sparse raw images, this is absolutely devastating. Reading them >>>> now takes more than (ext4) or nearly (xfs) twice as much time as reading >>>> a fully allocated image. So much for "if a filesystem driver has any >>>> sense". >> Are you sure that only the filesystem is the problem? Checking for every >> single byte of an image whether it is zero has to cost some performance. > Well, yes, but "read data location from FS metadata" + "realize it's a > hole" + memset() + "repe scasb" shouldn't take twice as much time as > "read data location from FS metadata" + "read data from SSD". > > I expected the "realize it's a hole" part to fall out for free, so this > would that memset() + repe scasb take much longer than reading data from > the SSD -- and that's just pretty much impossible. >
This makes a lot of sense, but just to double-check, what does profiling say? Paolo
