On Mon, 3 Mar 2025, Christoph Hellwig wrote: > On Mon, Mar 03, 2025 at 05:16:48PM +0100, Mikulas Patocka wrote: > > What should I use instead of bmap? Is fiemap exported for use in the > > kernel? > > You can't do an ahead of time mapping. It's a broken concept. Swapfile does ahead of time mapping. And I just looked at what swapfile does and copied the logic into dm-loop. If swapfile is not broken, how could dm-loop be broken? > > Would Jens Axboe agree to merge the dm-loop logic into the existing loop > > driver? > > What logic? The ahead-of-time mapping. > > Dm-loop is significantly faster than the regular loop: > > > > # modprobe brd rd_size=1048576 > > # dd if=/dev/zero of=/dev/ram0 bs=1048576 > > # mkfs.ext4 /dev/ram0 > > # mount -t ext4 /dev/ram0 /mnt/test > > # dd if=/dev/zero of=/mnt/test/test bs=1048576 count=512 > > All of this needs to be in a commit log. Also note that the above: > > a) does not use direct I/O which any sane loop user should > b) is not on a file but a block device, rendering the use of a loop > device entirely pointless. With "losetup --direct-io=on /dev/loop0 /mnt/test/test", it is even slower than without: READ: bw=217MiB/s (227MB/s), 217MiB/s-217MiB/s (227MB/s-227MB/s), io=2170MiB (2275MB), run=10003-10003msec WRITE: bw=217MiB/s (227MB/s), 217MiB/s-217MiB/s (227MB/s-227MB/s), io=2169MiB (2274MB), run=10003-10003msec with --direct-io=off READ: bw=398MiB/s (417MB/s), 398MiB/s-398MiB/s (417MB/s-417MB/s), io=3978MiB (4171MB), run=10003-10003msec WRITE: bw=398MiB/s (417MB/s), 398MiB/s-398MiB/s (417MB/s-417MB/s), io=3981MiB (4175MB), run=10003-10003msec Mikulas
