Performance testing with bcache

[Andrew Martin <amartin@xes-inc.com>]

Hello,

I am working on setting up bcache on Ubuntu 13.10 (with kernel 3.11.0-17) 
for read performance testing in writethrough mode. I've written a test script 
which uses dd to write data to the bcache device, read it back in, and then 
attempt to read it again (to measure cache speed):
# write the data to the disk
dd if=/dev/zero of=/path/to/bcache/mount/file.raw bs=512 count=2

# read the data (using iflag=nocache to make sure it gets into bcache's 
# cache and doesn't use the kernel's cache) 
dd if=/path/to/bcache/mount/file.raw of=/dev/null iflag=nocache

# now perform the read, which should read from bcache's cache
dd if=/path/to/bcache/mount/file.raw of=/dev/null iflag=nocache

However, if I cat /sys/block/bcache0/bcache/cache/cache_available_percent
before and after performing the read, the value is always 100. Would using
iflag=direct be better than iflag=nocache in this case? Is there a better
way to force data into the cache for this type of performance benchmarking?

Thanks,

Andrew Martin