MacOS – OS X lion and SSD

macosssd

I recently got a Crucial M4 SSD for my mid-2009 MacBook Pro 13" (2.53 GHz). Since I paid some money, I would like to get the best performance possible out of it. Running a speed test using app store Black Magic Speed Test, it showed 260 Mb read and 170 Mb write, although on the Crucial website it says read up to 500 Mb. Trim is not enabled and firmware is the latest (0309).

The connection speed is SATA 3 GB/s.

Is this performance normal or something is going wrong?

From system profiler :

Vendor: NVidia
 Product:   MCP79 AHCI
 Link Speed:    3 Gigabit
 Negotiated Link Speed: 3 Gigabit
 Description:   AHCI Version 1.20 Supported

   M4-CT128M4SSD2:

      Capacity: 128.04 GB (128,035,676,160 bytes)
      Model:    M4-CT128M4SSD2                          
      Revision: 309
      Serial Number:    000000001204032BCB4A
      Native Command Queuing:   Yes
      Queue Depth:  32
      Removable Media:  No
      Detachable Drive: No
      BSD Name: disk0
      Medium Type:  Solid State
      TRIM Support: No
      Partition Map Type:   GPT (GUID Partition Table)
      S.M.A.R.T. status:    Verified
      Volumes:
    disk0s1:
      Capacity: 209.7 MB (209,715,200 bytes)
      BSD Name: disk0s1
      Content:  EFI
    Macintosh HD:
      Capacity: 127.18 GB (127,175,917,568 bytes)
      Available:    63.01 GB (63,009,751,040 bytes)
      Writable: Yes
      File System:  Journaled HFS+
      BSD Name: disk0s2
      Mount Point:  /
      Content:  Apple_HFS
    Recovery HD:
      Capacity: 650 MB (650,002,432 bytes)
      BSD Name: disk0s3
      Content:  Apple_Boot

Best Answer

Your SSD's link speed is limited by the troughput of the SATA interface and the protocol overhead.

  • SATA 3 (6 Gbit/s): max. 600MB/s
  • SATA 2 (3 Gbit/s): max. 300MB/s
  • SATA 1 (1.5 Gbit/s): max. 150MB/s

A document by the Serial ATA International Organisation says:

What’s the real-world data transfer rate of SATA 6Gb/s?

Answer:
The realizable transfer rate across a 6Gb/s SATA link depends on the efficiency of the controller design on both the host and device sides of the interconnect. The SATA 6Gb/s interface transmits information at 600MB/s, however not all 600MB/s are realized as the user data payload because the protocol includes other data and handshaking communications between the host and device. In general, the SATA interface is very efficient. Realized transfer rates are typically very close to the theoretical maximum, which is one of the primary benefits of SATA technology for mass storage devices.

What overhead brings the 6Gb/s transfer rate down to the real-world throughput?

Answer:
There are two general categories of overhead that come into play: a communication used to send commands and receive status, and a low-level communication that handles handshakes between the host and the devices to assure the integrity of the transmission.

It's best to compare your throughput rates with that of other Crucial M4 users. An optimal comparison would compare disks using the same firmware, benchmark tool and SATA interface.

However, the best comparison I found is a thread on forum.crucial.com. In this thread users are comparing Crucial M4 128GB (firmware 009) speeds on a SATA 2 interface. Even though you are using a different firmware, the results should be somewhat comparable because the firmware update 0309 was not aimed improving transfer rates. Judging by this thread, I'd say that your throughput is fine.

The only way to get the promised speed of the Crucial M4 is by using a SATA 3 interface. In my MBP I'm currently using a Crucial M4 128GB (firmware 0309) with a SATA 3 interface. The Black Magic Speed Test gives me:

  • max. 510 MB/s read
  • max. 190 MB/s write
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