Poor performance with bcache write-back mode

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Hi all,

I have installed Linux kernel 3.9.2 in my Fedora 18.  When I ran FIO in bcache write-through mode, the performance was good, but running a similar test in bcache write-back mode, I got very poor performance.
Did I miss any options when setting the write-back mode?

Environment
===========
500G backing store at /dev/sdb1
150G SSD at /dev/sdc1

Write-through mode
==================
make-bcache -B /dev/sdb1 -C /dev/sdc1
echo /dev/sdb1 > /sys/fs/bcache/register
echo /dev/sdc1 > /sys/fs/bcache/register
mkfs.ext4 /dev/bcache0
mount /dev/bcache0 /mnt/bcache


Running FIO with random, 100% read, iosize=4096, queue depth=16
the result is approx. 65k IOPS


Write-backe mode (setup is almost the same as write-through except the extra step at the end "echo writeback ...")
================
make-bcache -B /dev/sdb1 -C /dev/sdc1
echo /dev/sdb1 > /sys/fs/bcache/register
echo /dev/sdc1 > /sys/fs/bcache/register
mkfs.ext4 /dev/bcache0
mount /dev/bcache0 /mnt/bcache
echo writeback > /sys/block/bcache0/bcache/cache_mode

Running FIO with random, 100% read, iosize=4096, queue depth=16
the result is approx. 12k IOPS

Running FIO with random, 80% read 20% write, iosize=4096, queue depth=16
the result is approx. 2k IOPS

Thanks,
Patrick 
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