After upgrading a POWER9 pSeries guest on a PowerNV host from Linux 6.1.0-18 to 6.12.94, rebuilding a degraded RAID10 array behind an LSI SAS3008 controller passed through via VFIO proceeds at ~1MB/s with 100% CPU load on one core. Booting the same guest with the old 6.1 series kernel and resuming the rebuild of the same array proceeds at over 200MB/s with minimal CPU load. For completeness, the default 6.12 readahead settings were adjusted back down to the same values as the 6.1 kernel, with no improvement observed.
No variation in the regression was found between host kernel 6.12 and host kernel 6.18. Since there are no other reports of similar issues from amd64 users, it is presumed this is a POWER-specific regression.
