Hi, I see an approximately 23% reduction in performance through a loop device. Is it expected? This is kernel 5.5.3. The setup is SSD, plain partitions, no LVM or encryption, Btrfs. This scrub performance is typical. $ sudo btrfs scrub status / UUID: b8e290d5-1dc5-429f-8201-10ca5b2c0b95 Scrub started: Sun Feb 16 19:39:01 2020 Status: finished Duration: 0:00:54 Total to scrub: 28.00GiB Rate: 531.06MiB/s Error summary: no errors found [chris@fmac ~]$ On this file system is a sparse file, chattr +C is set, and it's attached to /dev/loop0 and mounted at /mnt. $ sudo btrfs scrub status /mnt UUID: 63a7e2b9-6a5e-4e94-9cc9-f90d01de7541 Scrub started: Sun Feb 16 20:06:51 2020 Status: finished Duration: 0:00:13 Total to scrub: 5.15GiB Rate: 405.79MiB/s Error summary: no errors found $ I don't think file system over accounts for much more than a couple percent of this, so I'm curious where the slow down might be happening? The "hosting" Btrfs file system is not busy at all at the time of the loop mounted filesystem's scrub. I did issue 'echo 3 > /proc/sys/vm/drop_caches' before the loop mount image being scrubbed, otherwise I get ~1.72GiB/s scrubs which exceeds the performance of the SSD (which is in the realm of 550MiB/s max) Thanks, -- Chris Murphy
