Samsung SSD 840 speed issue Can any one explain me this ? I re-install BlackMagic but it ain't change a thing.
I'm running 10.9.3 and there is only one partition on the SSD.

With the terminal :
MBP-Thomas in ~
○ → dd if=/dev/zero bs=1024k of=tstfile count=1024
1024+0 records in
1024+0 records out
1073741824 bytes transferred in 35.587628 secs (30171773 bytes/sec)

 MBP-Thomas in ~
○ → dd if=tstfile  of=/dev/zero 
2097152+0 records in
2097152+0 records out
1073741824 bytes transferred in 11.061699 secs (97068437 bytes/sec)

 MBP-Thomas in ~
○ → cd /

 MBP-Thomas in /
○ → sudo dd if=/dev/zero bs=1024k of=tstfile count=1024
Password:
1024+0 records in
1024+0 records out
1073741824 bytes transferred in 33.653468 secs (31905830 bytes/sec)

 MBP-Thomas in /
○ → dd if=tstfile  of=/dev/zero 
2097152+0 records in
2097152+0 records out
1073741824 bytes transferred in 11.582934 secs (92700332 bytes/sec)

Which is also pretty bad...
Result of iotop :
MBP-Thomas in ~
○ → sudo iotop -C 5 12
Password:
Tracing... Please wait.
2014 May 29 12:07:16,  load: 1.38,  disk_r:      4 KB,  disk_w:   9124 KB

  UID    PID   PPID CMD              DEVICE  MAJ MIN D            BYTES
  501    284    277 cfprefsd         ??        1   2 R             4096
  501    284    277 cfprefsd         ??        1   2 W            16384
  501   3905     46 lssave           ??        1   2 W          9326592

2014 May 29 12:07:21,  load: 1.43,  disk_r:      0 KB,  disk_w:      0 KB

  UID    PID   PPID CMD              DEVICE  MAJ MIN D            BYTES

2014 May 29 12:07:26,  load: 1.40,  disk_r:     40 KB,  disk_w:   1960 KB

  UID    PID   PPID CMD              DEVICE  MAJ MIN D            BYTES
    0      1      0 launchd          ??        1   2 R            40960
    0      1      0 launchd          ??        1   5 W            40960
    0      0      0                  ??        1   5              74752
    0      0      0 kernel_task      ??        1   2 W           888832
    0      1      0 launchd          ??        1   2 W          1077248
    0      0      0                  ??        1   2            1200128

2014 May 29 12:07:31,  load: 1.36,  disk_r:      0 KB,  disk_w:     12 KB

  UID    PID   PPID CMD              DEVICE  MAJ MIN D            BYTES
  501    352    296 node-webkit      ??        1   2 W            12288

2014 May 29 12:07:36,  load: 1.33,  disk_r:      0 KB,  disk_w:      0 KB

  UID    PID   PPID CMD              DEVICE  MAJ MIN D            BYTES

2014 May 29 12:07:41,  load: 1.31,  disk_r:      0 KB,  disk_w:      0 KB

Update :
Ok. Now it says that writing speed is 27.4MB/s (27379034 bytes/sec) and reading speed is 4.45GB/s (4499797299 bytes/sec)
WTF ?
I have also experienced slow performance on an SSD and it was due to other active processes writing to the disk continously.

Have a look at question on superuser on how to monitor OSX disk activity: https://superuser.com/questions/89266/whats-causing-all-the-disk-activity-on-os-x

Basically run this either this command form Terminal:

iotop -C 5 12


or this other:

fs_usage


To locate any process that is writing to the disk continously.

Another thing to try is booting from a clean install of OSX on an external USB disk, and the benchmark the internal SSD.

I would also determine if the SDD is running the latest Samsung firmware. You can download the latest version from this Samsung support page: http://www.samsung.com/global/business/semiconductor/samsungssd/downloads.html

Beware that it is recommended to have a backup of the data before attempting to update the firmware.