Re: Poor performance with bcache write-back mode

[Patrick Ng <ppng-/E1597aS9LQAvxtiuMwx3w@public.gmane.org>]

Kent Overstreet <koverstreet@...> writes:

> 
> On Thu, May 16, 2013 at 02:51:46PM -0700, Patrick Ng wrote:
> > 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
> 
> You're probably getting bit by the congested bypass - try setting
> congested_read_threshold_us and congested_write_threshold_us both to 0.
> 
> I wish there was a better solution to the congestion bypass stuff, it's
> a hard problem...
> 

Hi Kent, thank you for responding to my question.

I have tried setting both congested_read_threshold_us and 
congested_write_threshold_us to zero.

The result of FIO random 100% read test in WriteBack mode has improved and 
the IOPS is as good as in WriteThur mode.

The result of FIO random 80% read 20% write remain the same with no 
improvement.

It appears that the option does not help on the WRITE?

Thanks,
Patrick