bad bcache performance - crappy SSD?

Hi,

so far I have been unable to get an actual performance improvement out of bcache, and I suspect that my SSD is just crap. Any help or hint appreciated. What (SATA) SSDs have you been using successfully with bcache?

My host system: Ubuntu 14.04 x64.
kernels tried: Ubuntu stock (3.13.0-24), 3.14, 3.15-rc{5,6}.

storage structure:
3xSATA (sda,sdb,sdc) => md raid5 (md125) => bcache0 => LVM (dm-1)

benchmark: install linux kernel header package inside kvm guest

cache device not attached: 0:18 minutes
SSD cache device attached (writethrough): 3:27 minutes
SSD cache device attached (writeback): 3:25 minutes

With attached cache device, the ssd seems very busy (iostat %util), while the raid members are not:

     Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
(5) sdc               0.00    12.33    0.00   57.33     0.00    57.00     1.99     0.06    1.07    0.00    1.07   0.79   4.53
(5) sdb               0.33    12.00    0.33   57.00     2.67    54.33     1.99     0.07    1.19    0.00    1.19   0.84   4.80
(5) sda               0.00    11.33    0.00   57.00     0.00    51.67     1.81     0.06    1.10    0.00    1.10   0.80   4.53
(*) sdf               0.00     0.00    0.00   54.67     0.00    27.17     0.99     0.98   18.05    0.00   18.05  17.95  98.13

     (5) raid 5 member;  (*) SSD

when I replace the SSD with a single SATA disk:
SATA cache device attached (writethrough): 0:19 minutes
SATA cache device attached (writeback): 0:13 minutes

The SSD is an Intel 530, has current firmware and plenty of DISCARDed blocks. Specs claim >= 13.000 incompressible writes/sec for the 120GB model, but the above data suggests it can handle just about 60.

=== START OF INFORMATION SECTION ===
Device Model: INTEL SSDSC2BW120A4
Serial Number: CVDA3393028N1207GN
LU WWN Device Id: 5 5cd2e4 04b82aa5a
Firmware Version: DC32
User Capacity: 120,034,123,776 bytes [120 GB]
Sector Size: 512 bytes logical/physical
Rotation Rate: Solid State Device
Device is: Not in smartctl database [for details use: -P showall]
ATA Version is: ACS-2 (minor revision not indicated)
SATA Version is: SATA 3.0, 6.0 Gb/s (current: 3.0 Gb/s)
Local Time is: Sat May 24 20:37:14 2014 CEST
SMART support is: Available - device has SMART capability. SMART support is: Enabled

Regards
Matthias
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