Slow Write Performance w/ No Cache Enabled and Different Size Drives

[Adam Brenner <adam@aeb.io>]

Howdy,

I recently setup a new BTRFS filesystem based on BTRFS version 3.12 on
Linux kernel 3.13-1 running Debian Jessie.

The BTRFS volume spans 3x 4TB disks, two of which are using the entire
raw block device, and one of them is using a partition (OS disks). The
setup is like so:


     root@gra-dfs:/data/tmp# btrfs filesystem show
     Label: none  uuid: 63d51c9b-f851-404f-b0f2-bf84d07df163
	    Total devices 3 FS bytes used 3.03TiB
	    devid    1 size 3.61TiB used 1.01TiB path /dev/sda3
	    devid    2 size 3.64TiB used 1.04TiB path /dev/sdb
	    devid    3 size 3.64TiB used 1.04TiB path /dev/sdc

     Btrfs v3.12
     root@gra-dfs:/data/tmp# btrfs filesystem df /data
     Data, single: total=3.07TiB, used=3.03TiB
     System, RAID1: total=8.00MiB, used=352.00KiB
     System, single: total=4.00MiB, used=0.00
     Metadata, RAID1: total=5.00GiB, used=3.60GiB
     Metadata, single: total=8.00MiB, used=0.00
     root@gra-dfs:/data/tmp#
     root@gra-dfs:/home# mount | grep /data
     /dev/sda3 on /data type btrfs (rw,noatime,space_cache)
     root@gra-dfs:/home#


The setup is supposed to be "RAID-0 like" but with different size drives
within the volume, I created the BTRFS filesystem using the following
command based on the WiKi[1]

     mkfs.btrfs -d single /dev/sda3 /dev/sdb /dev/sdc -f

Once setup, I transferred roughly 3.1TB of data and noticed the write 
speed was limited to 200MB/s. This is the same write speed that I would 
see across a single device. I used dd with oflag=direct and a block size 
of 1M and a count of 1024 from /dev/zero. Both showed the same speeds.

So my question is, should I have setup the BTRFS filesystem with -d 
raid0? Would this have worked with multiple devices with different sizes?

[1]: 
https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices


-- 
Adam Brenner <adam@aeb.io>