On Tue, May 10, 2016 at 09:46:36AM -0600, Eric Blake wrote: > On 05/10/2016 09:41 AM, Alex Bligh wrote: > > > > On 10 May 2016, at 16:29, Eric Blake <[email protected]> wrote: > > > >> So the kernel is currently one of the clients that does NOT honor block > >> sizes, and as such, servers should be prepared for ANY size up to > >> UINT_MAX (other than DoS handling). > > > > Interesting followup question: > > > > If the kernel does not fragment TRIM requests at all (in the > > same way it fragments read and write requests), I suspect > > something bad may happen with TRIM requests over 2^31 > > in size (particularly over 2^32 in size), as the length > > field in nbd only has 32 bits. > > > > Whether it supports block size constraints or not, it is > > going to need to do *some* breaking up of requests. > > Does anyone have an easy way to cause the kernel to request a trim > operation that large on a > 4G export? I'm not familiar enough with > EXT4 operation to know what file system operations you can run to > ultimately indirectly create a file system trim operation that large. > But maybe there is something simpler - does the kernel let you use the > fallocate(2) syscall operation with FALLOC_FL_PUNCH_HOLE or > FALLOC_FL_ZERO_RANGE on an fd backed by an NBD device? >
It was fairly reproducible here, we just used a random qcow2 image with some Debian minimal system pre-installed, mounted that qcow2 image through qemu-nbd then compiled a whole kernel inside it. Then you can make clean and run fstrim on the mount point. I'm assuming you can go faster than that by just writing a big file to the qcow2 image mounted without -o discard, delete the big file, then remount with -o discard + run fstrim. Quentin ------------------------------------------------------------------------------ Mobile security can be enabling, not merely restricting. Employees who bring their own devices (BYOD) to work are irked by the imposition of MDM restrictions. Mobile Device Manager Plus allows you to control only the apps on BYO-devices by containerizing them, leaving personal data untouched! https://ad.doubleclick.net/ddm/clk/304595813;131938128;j _______________________________________________ Nbd-general mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/nbd-general
