Re: bad bcache performance - crappy SSD?

[Peter Kieser <peter@kieser.ca>]

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What filesystem are you running on top of bcache? There are issues with 
XFS and barrier modes causing 100% util on the cache device.

-Peter

On 2014-05-26 3:16 PM, Matthias Ferdinand wrote:
> 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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