I bought a Late 2013 iMac with new generation PCIe Samsung XP941 M.2 based SSD 256Gb. Really powerful SSD. But of course, I was scared about SSD rewrite cycles and wanted to check it because I want to keep this computer for 5 years. Using an app called DriveDx, which can check SSD and HDD lifetime and other SMART attributes, I generated this log:

SSD Lifetime Left Indicator : GOOD 95.0%
 5 Retired Block Count     0    100% OK
173 Wear Leveling Count    0x100990005  95.0% OK
174 Unexpected Power Loss Count   0xA368E 99.0% OK
192 Unsafe Shutdown Count    6    99.0% OK

I used this Mac for 3 days. I just did clean install of OS X, installed 80 GB of software on it, and installed Windows in the second partition and 95% of wear leveling count?!

My SSD has 2 partitions: Mac OS X (133 GB) and second for Windows 8.1 (117 GB) I know that OS X is very optimized to SSD, but I have second Windows partition and don’t know how two systems will live together. Need Windows ONLY for gaming. I have two question.

  1. I heard that rewrite cycles depends on SSD size. Is it true that my drive turned into two 128 Gb SSD? When one of OS reach 128 Gb then it will rewrite cells again. Right? And lifetime will be a half of 256 Gb. And how about TRIM with two different systems. Is it working correctly?

  2. I have an idea how to reduce read/write on Windows. Want to make 30Gb partition for Windows and install all games in my external HDD USB 3.0. Yeah, game loading will be slower, but less read/write gaming data. But this method is useless if external HDD use system SSD as temp storage between RAM and HDD. Does Windows use system drive to store some temporary data between system drive and external HDD or it has direct route to RAM from HDD? If it does, using external HDD is useless, because all temp data will be passing through system SSD and make rewrite.

1 Answer 1


Memory wear corresponds more to drive usage than to drive size. SSD controller employs wear leveling techniques to spread writes evenly over the whole SSD, so the wear level is approximately the same for all memory cells during drive’s lifetime. Assumption about wear–size relation comes from premise that larger drive will probably contain more unallocated space which could be used for wear leveling. Needless to say that this does not hold universally.

Trim command on the other hand is issued by operating system, which notifies SSD which pages no longer contain valid data, and recent versions of both Windows and OS X supports it (although Apple does not enable it by default on 3ʳᵈ party SSD, so one has to use Trim enabler or similar application to get it working.

To answer your questions:

  1. No, wear leveling does not care about partition scheme. Yes, Trim will work correctly.

  2. Windows does memory caching of drive operations, yet it happens in RAM and does not involve other drives.

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