UBIFS performance with no_chk_data_crc option

[Reuben Dowle]

It seems to me that enabling the no_chk_data_crc option actually causes
SLOWER read performance. I get these results:

umount NOS
mount -t ubifs -o ro,no_chk_data_crc /usr/share/NOS/
echo 3 > /proc/sys/vm/drop_caches
time cat NOS/charts/Navico/ShadedRelief/_3DTexture_1024_0_World.at5 >
/dev/null
real    0m 17.55s
user    0m 0.00s
sys     0m 17.53s

umount NOS
mount -t ubifs -o ro,chk_data_crc /usr/share/NOS/
echo 3 > /proc/sys/vm/drop_caches
time cat NOS/charts/Navico/ShadedRelief/_3DTexture_1024_0_World.at5 >
/dev/null
real    0m 12.12s
user    0m 0.01s
sys     0m 12.09s

I also check what the performance is with defaults:

umount NOS
echo 3 > /proc/sys/vm/drop_caches
time cat NOS/charts/Navico/ShadedRelief/_3DTexture_1024_0_World.at5 >
/dev/null
real    0m 17.90s
user    0m 0.01s
sys     0m 17.87s

So it seems:
- defaults (documentation says this is checking CRC) -> slow
- no crc checks -> slow
- explicitely enabled crc checks -> fast

This seems very repeatable. I get the same numbers every time. I think
perhaps the logic of the mount options is reversed?

Regards,
Reuben Dowle
Design Engineer
Navico Asia Pacific