overprovisioning

Overprovisioning

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One of the most interesting things to consider when grabbing an SSD for your Power Mac G5 is the concept of overprovisioning.

It’s a confusing concept – but here it is in a nutshell.

Hard drives and SSDs don’t really have a delete command. Instead, when the operating system is told to delete a file, it marks a table saying this place in the hard drive is empty. For traditional platter hard drives, this just meant the OS telling the hard drive to write the data in that certain place. Everybody was happy.

But with SSDs, things get tricky. SSDs use NAND flash memory which cannot be easily written over like traditional hard drives. They must be first erased for the new data to be able to be written. So, theoretically, a SSD could fill up to 100% capacity and not be able to write any new data at all, even your operating system caches and page files and what not. When that happens, the SSD takes a serious nose dive in performance. To counter this, SSDs tend to have support for things like TRIM, which tells the SSD to put certain spots of the drive into some sort of garbage collection. Quietly in the background, the SSD will erase those blocks that are empty so data can be written and read at a normal, brisk pace.

More good news – almost all SSDs are built with some overprovisioning, meaning that some space of the drive is set aside as clean empty space for things to get shifted about when stuff is getting deleted and organized.

Here’s the bad news – Power Mac G5s do not have TRIM support via our now dated operating system choices of Tiger or Leopard. Without TRIM, your SSD bears the risk of running ragged in its performance, especially if you max out the drive with data. With every block having been written (if you do max it out), there is no mechanism to tell the SSD to free up deleted space even if you get rid of all of those apps, movies, games, and so on. This is the problem of using a SSD in a system without TRIM support.

In response, it is possible to overprovision even more than factory standard. You do this by first deleting the drive via a secure erase in Disk Utility. Then you create a partition that is smaller than the capacity of the drive. I hear 10-20% is often recommended. A 240GB drive might then look like a 220GB drive on your Mac. The extra space is then seen as not used by the operating system and always empty. Even when your SSD gets close to full, there will still be free space for the SSD to shift things around and keep its somewhat zippy performance up. If you do fill the hard drive to the brim, you may still see performance degradation but it may not be as severe.

There is a lot of debate about this. Many SSDs have some other built-in features to counter a lack of trim. Sandforce drives in particular have some pretty aggressive garbage collection, so you shouldn’t see degradation at all unless you really just torture that drive. That’s why I’ve seen Sandforce drives as the top recommends for G5s, especially the Intel 520/530 series. Even without overprovisioning (beyond factory settings) or TRIM, it’s likely that you may never see performance degradation for a long time. By a year or two, it might be time to upgrade your SSD anyway, right?

Here’s some additional reading:

– Nathan