[Google Pixel Questions & Answers] cnkhanh:
Here is the cmd lines when installing 8.1, seems fine with me.

target reported max download size of 536870912 bytes

sending 'bootloader_a' (32248 KB)...

OKAY [ 0.888s]

writing 'bootloader_a'...

(bootloader) Valid bootloader version.

(bootloader) Flashing active slot "_a"

(bootloader) Flashing active slot "_a"

OKAY [ 2.059s]

finished. total time: 2.951s

rebooting into bootloader...

OKAY [ 0.051s]

finished. total time: 0.052s

target reported max download size of 536870912 bytes

sending 'radio_a' (57320 KB)...

OKAY [ 1.499s]

writing 'radio_a'...

OKAY [ 0.486s]

finished. total time: 1.989s

rebooting into bootloader...

OKAY [ 0.050s]

finished. total time: 0.051s

extracting android-info.txt (0 MB) to RAM...

extracting boot.img (28 MB) to disk... took 0.410s

target reported max download size of 536870912 bytes

archive does not contain 'boot.sig'

archive does not contain 'boot_other.img'

archive does not contain 'dtbo.img'

archive does not contain 'dt.img'

archive does not contain 'recovery.img'

extracting system.img (1751 MB) to disk... took 18.393s

archive does not contain 'system.sig'

extracting system_other.img (551 MB) to disk... took 6.660s

archive does not contain 'system.sig'

archive does not contain 'vbmeta.img'

extracting vendor.img (250 MB) to disk... took 2.662s

archive does not contain 'vendor.sig'

archive does not contain 'vendor_other.img'

wiping userdata...

mke2fs 1.43.3 (04-Sep-2016)

Creating filesystem with 6509568 4k blocks and 1630208 inodes

Filesystem UUID: 657305e6-e288-11e7-8b52-538b74cf9ac7

Superblock backups stored on blocks:

32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,

4096000

Allocating group tables: done

Writing inode tables: done

Creating journal (32768 blocks): done

Writing superblocks and filesystem accounting information: done

--------------------------------------------

Bootloader Version...: 8996-012001-1710040120

Baseband Version.....: 8996-130091-1710201747

Serial Number........: FA6960306426

--------------------------------------------

checking product...

OKAY [ 0.050s]

checking version-bootloader...

OKAY [ 0.048s]

checking version-baseband...

OKAY [ 0.049s]

sending 'boot_a' (28945 KB)...

OKAY [ 0.750s]

writing 'boot_a'...

OKAY [ 0.296s]

erasing 'system_a'...

OKAY [ 0.320s]

sending sparse 'system_a' 1/4 (524284 KB)...

OKAY [ 13.651s]

writing 'system_a' 1/4...

OKAY [ 3.451s]

sending sparse 'system_a' 2/4 (524284 KB)...

OKAY [ 13.604s]

writing 'system_a' 2/4...

OKAY [ 3.488s]

sending sparse 'system_a' 3/4 (524284 KB)...

OKAY [ 14.015s]

writing 'system_a' 3/4...

OKAY [ 3.446s]

sending sparse 'system_a' 4/4 (220660 KB)...

OKAY [ 5.700s]

writing 'system_a' 4/4...

OKAY [ 1.494s]

erasing 'system_b'...

OKAY [ 0.261s]

sending sparse 'system_b' 1/2 (524284 KB)...

OKAY [ 13.702s]

writing 'system_b' 1/2...

OKAY [ 3.451s]

sending sparse 'system_b' 2/2 (40660 KB)...

OKAY [ 1.083s]

writing 'system_b' 2/2...

OKAY [ 0.355s]

erasing 'vendor_a'...

OKAY [ 0.142s]

sending 'vendor_a' (257004 KB)...

OKAY [ 6.358s]

writing 'vendor_a'...

OKAY [ 1.758s]

Setting current slot to 'a'...

OKAY [ 0.088s]

erasing 'userdata'...

OKAY [ 1.705s]

sending 'userdata' (4412 KB)...

OKAY [ 0.196s]

writing 'userdata'...

OKAY [ 0.100s]

rebooting...

finished. total time: 89.845s

Press any key to exit...