[Bug 238733] kvm disk i/o extremely slow

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=238733

--- Comment #5 from Alexander Shikov <a.shikov@itcons.net.ua> ---
Hello!

It seems I have the same issue with 13.1-RELEASE running as guest in Proxmox
6.4-15.

vtblk0: <VirtIO Block Adapter> on virtio_pci1
vtblk0: 40960MB (83886080 512 byte sectors)

# dd if=/dev/random of=test.dat bs=1M count=100 iflag=fullblock
100+0 records in
100+0 records out
104857600 bytes transferred in 14.858133 secs (7057253 bytes/sec)

sysctl kern.timecounter.hardware HPET or TSC-low does not affect the issue.

Linux Mint on the same host machine:
# dd if=/dev/urandom of=test.dat bs=1M count=100 iflag=fullblock
100+0 records in
100+0 records out
104857600 bytes (105 MB, 100MiB) copied, 0,588288 s, 178 MB/s


If switch VirtIO Block to SATA in VM configuration then speed changes to
better:
ada0 at ahcich0 bus 0 scbus2 target 0 lun 0
ada0: <QEMU HARDDISK 2.5+> ATA-7 SATA device
ada0: Serial Number QM00005
ada0: 150.000MB/s transfers (SATA 1.x, UDMA5, PIO 8192bytes)
ada0: Command Queueing enabled
ada0: 40960MB (83886080 512 byte sectors)

# dd if=/dev/random of=test.dat bs=1M count=100 iflag=fullblock
100+0 records in
100+0 records out
104857600 bytes transferred in 0.354972 secs (295396745 bytes/sec)

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