[How To] SSD over-provisioning

Definition:

Over-provisioning (sometimes spelled as OP, over provisioning, or overprovisioning) is the difference between the physical capacity of the flash memory and the logical capacity presented through the operating system (OS) as available for the user. During the garbage collection, wear-leveling, and bad block mapping operations on the SSD, the additional space from over-provisioning helps lower the write amplification when the controller writes to the flash memory.

http://en.wikipedia.org/wiki/Write_amplification#Over-provisioning

 

There are two possible ways of enabling over-provisioning SSDs by the end-user:

1. Simple (software level)

Windows/Linux/Mac: Just create partitions which will use less than the maximum available capacity of the SSD. The rest of the space should show up as "unused".

The SSD’s controller will know how to use this "unused" space for it’s own tasks.

 

2. Advanced (hardware level)

You can use the Linux hdparm utility to set your overprovisioning parameters in this fashion:

root@ubuntu-10-10:~# hdparm -N /dev/sdb

/dev/sdb:
 max sectors   = 312581808/312581808, HPA is disabled

root@ubuntu-10-10:~# hdparm -Np281323627 /dev/sdb

/dev/sdb:
 setting max visible sectors to 281323627 (permanent)
Use of -Nnnnnn is VERY DANGEROUS.
You have requested reducing the apparent size of the drive.
This is a BAD idea, and can easily destroy all of the drive’s contents.
Please supply the --yes-i-know-what-i-am-doing flag if you really want this.
Program aborted.

root@ubuntu-10-10:~# hdparm -Np281323627 --yes-i-know-what-i-am-doing /dev/sdb

/dev/sdb:
 setting max visible sectors to 281323627 (permanent)
 max sectors   = 281323627/312581808, HPA is enabled

root@ubuntu-10-10:~# 

According to the Intel docs, it would be enough simply to keep the sectors in a "clean" state (in which they would be after a secure erase) by never writing to them. If you define a container/volume which only ever uses 300 GB of each of your drives, this is exactly what you are doing for half of your drives’ sectors.

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