Linux 5.10.8 Kernel Released - Finally Fixes That Btrfs Performance Regression
Written by Michael Larabel in Linux Kernel on 17 January 2021 at 01:24 PM EST. 14 Comments
LINUX KERNEL --
Linux 5.10.8 is out today as the latest stable release for the Linux 5.10 LTS series. Making this point release notable is that it finally addresses the 5.10 Btrfs performance regression.

As noted back on Christmas, Linux 5.10 was seeing significant slowdowns on Btrfs. For simply unpacking a Linux kernel source .tar.zst file it could easily take multiple times longer on this stable kernel version.

While patches for addressing this poor Btrfs behavior on Linux 5.10 were floating around since before the end of the year, it's taken until now to get them tested and queued up for mainline integration. Linux 5.11 meanwhile has a plethora of Btrfs improvements.

With shrinking delalloc pages instead of full inodes the main issue should be addressed. As for the original cause, "this introduced a pretty serious performance regression. To reproduce the user untarred the source tarball of Firefox (360MiB xz compressed/1.5GiB uncompressed), and would see it take anywhere from 5 to 20 times as long to untar in 5.10 compared to 5.9. This was observed on fast devices (SSD and better) and not on HDD. The root cause is because before we would generally use the normal writeback path to reclaim delalloc space, and for this we would provide it with the number of pages we wanted to flush. The referenced commit changed this to flush that many inodes, which drastically increased the amount of space we were flushing in certain cases, which severely affected performance."

Overall things should be in good shape with Linux 5.10.8. Btrfs developer Josef Bacik commented on that page, "5.10+patch is significantly faster than plain 5.9 because of my patch series "Change data reservations to use the ticketing infra" which contained the patch that introduced the regression, but generally improved the overall ENOSPC flushing mechanisms."

So all should be well now on Linux 5.10 LTS stable as of Linux 5.10.8. No other pressing regressions for this LTS kernel come to mind at this point.
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Michael Larabel is the principal author of Phoronix.com and founded the site in 2004 with a focus on enriching the Linux hardware experience. Michael has written more than 20,000 articles covering the state of Linux hardware support, Linux performance, graphics drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and OpenBenchmarking.org automated benchmarking software. He can be followed via Twitter or contacted via MichaelLarabel.com.

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