Re: [PATCH] the dm-loop target

Mikulas Patocka
2025-03-10T11:20:45Z

On Sun, 9 Mar 2025, Ming Lei wrote:

> On Fri, Mar 07, 2025 at 04:21:58PM +0100, Mikulas Patocka wrote:
> > > I didn't say you were. I said the concept that dm-loop is based on
> > > is fundamentally flawed and that your benchmark setup does not
> > > reflect real world usage of loop devices.
> >
> > > Where are the bug reports about the loop device being slow and the
> > > analysis that indicates that it is unfixable?
> >
> > So, I did benchmarks on an enterprise nvme drive (SAMSUNG
> > MZPLJ1T6HBJR-00007). I stacked ext4/loop/ext4, xfs/loop/xfs (using losetup
> > --direct-io=on), ext4/dm-loop/ext4 and xfs/dm-loop/xfs. And loop is slow.
> >
> > synchronous I/O:
> > fio --direct=1 --bs=4k --runtime=10 --time_based --numjobs=12 --ioengine=psync --iodepth=1 --group_reporting=1 --filename=/mnt/test2/l -name=job --rw=rw
> > raw block device:
> > READ: bw=399MiB/s (418MB/s), 399MiB/s-399MiB/s (418MB/s-418MB/s), io=3985MiB (4179MB), run=10001-10001msec
> > WRITE: bw=399MiB/s (418MB/s), 399MiB/s-399MiB/s (418MB/s-418MB/s), io=3990MiB (4184MB), run=10001-10001msec
> > ext4/loop/ext4:
> > READ: bw=223MiB/s (234MB/s), 223MiB/s-223MiB/s (234MB/s-234MB/s), io=2232MiB (2341MB), run=10002-10002msec
> > WRITE: bw=223MiB/s (234MB/s), 223MiB/s-223MiB/s (234MB/s-234MB/s), io=2231MiB (2339MB), run=10002-10002msec
> > xfs/loop/xfs:
> > READ: bw=220MiB/s (230MB/s), 220MiB/s-220MiB/s (230MB/s-230MB/s), io=2196MiB (2303MB), run=10001-10001msec
> > WRITE: bw=219MiB/s (230MB/s), 219MiB/s-219MiB/s (230MB/s-230MB/s), io=2193MiB (2300MB), run=10001-10001msec
> > ext4/dm-loop/ext4:
> > READ: bw=338MiB/s (355MB/s), 338MiB/s-338MiB/s (355MB/s-355MB/s), io=3383MiB (3547MB), run=10002-10002msec
> > WRITE: bw=338MiB/s (355MB/s), 338MiB/s-338MiB/s (355MB/s-355MB/s), io=3385MiB (3549MB), run=10002-10002msec
> > xfs/dm-loop/xfs:
> > READ: bw=375MiB/s (393MB/s), 375MiB/s-375MiB/s (393MB/s-393MB/s), io=3752MiB (3934MB), run=10002-10002msec
> > WRITE: bw=376MiB/s (394MB/s), 376MiB/s-376MiB/s (394MB/s-394MB/s), io=3756MiB (3938MB), run=10002-10002msec
> >
> > asynchronous I/O:
> > fio --direct=1 --bs=4k --runtime=10 --time_based --numjobs=12 --ioengine=libaio --iodepth=16 --group_reporting=1 --filename=/mnt/test2/l -name=job --rw=rw
> > raw block device:
> > READ: bw=1246MiB/s (1306MB/s), 1246MiB/s-1246MiB/s (1306MB/s-1306MB/s), io=12.2GiB (13.1GB), run=10001-10001msec
> > WRITE: bw=1247MiB/s (1308MB/s), 1247MiB/s-1247MiB/s (1308MB/s-1308MB/s), io=12.2GiB (13.1GB), run=10001-10001msec
>
> BTW, raw device is supposed to be xfs or ext4 over raw block device, right?

No - raw device means fio directly on /dev/nvme0n1

> Otherwise, please provide test data for this case, then it becomes one fair
> comparison because there should be lock contention for FS write IOs on same file.
>
> Thanks,
> Ming

Mikulas