Super User is a question and answer site for computer enthusiasts and power users. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

In case you're creating one partition, should it be as big as possible, or should a certain percentage of space be left unpartitioned?

share|improve this question
Why do you think a certain percentage should be left unused? – Steve Folly Mar 6 '10 at 13:55
I've heard stories that it might be needed for wear leveling. – Sander Rijken Mar 6 '10 at 15:13
up vote 5 down vote accepted

I recommend the former (as big as possible) to allow the drive controller to more efficiently utilize free blocks during file updates. Anand from AnandTech wrote an excellent article about that last year. (Note that the link goes to a part of the article where this is explained, but I recommend to read the whole article for gaining better understanding of the concept.)

should a certain percentage of space be left unpartitioned?

If you are worried about leaving some space for the drive controller to handle wear leveling, then you should not need to worry about that since most SSDs have space reserved for that already (i.e. 30GB drives can be 32GB large in reality).

share|improve this answer
I was indeed thinking about the wear levelling. In this case it's a harddisk advertized as 250GB, but I can partition almost 256GB. I wonder if it hurts to just max it out as far as Mac OSX allows – Sander Rijken Mar 6 '10 at 14:55
I see, this changes it a bit. I have no experience with OSX, so I'm not sure if the reported 256GB is the full size or the one without reserve space. I think that we can safely assume that this is not being a "KB vs kB problem" (if it would, I'd expect you to see 268GB instead 256GB), so I'm more inclined to 256GB being the full size of the drive (but I'm really heavily assuming here). Unless anyone else can advise better, I think that your best action would be to confirm this with the drive manufacturer. You can also try to ask this on if you already haven't done so. – MicE Mar 9 '10 at 21:09
I think I am asking this on – Sander Rijken Mar 11 '10 at 15:03
@Sander: Ha, right! I'm sorry, I don't what I was thinking then. I guess I was switching back and forth between StackOverflow and SuperUser and just lost track ;-) – MicE Mar 11 '10 at 17:46
@sander I believe MicE's advice is correct - you can (and should) safely use the entire 256GB. As AnandTech describe ( when the manufacturer advertises the drive as 250GB, there are actually 268GB (250*1024^3) on the drive. The drive's firmware appears to keep about 12GB of this "hidden" as spare area, leaving 256GB (256*10^9) or 238GB (238*1024^3) as user space. – sblair Mar 16 '10 at 23:39

As big as possible, unless you have a specific reason to leave a certain percentage unpartitioned.

share|improve this answer

As a general rule you want to format your SSD to 80% of the advertised size (the 20% rule stated in reverse) for performance reasons. discusses capacities and overprovisioning in regards to lifetime/reliability discusses free space effecting speed on Trim enabled drives:


Performance in a TRIM enabled system is now determined not by the number of invalid blocks on your SSD, but rather the amount of free space you have. I went into a deep explanation of the relationship between free space and the performance of some SSDs here.

TRIM will make sure that you don’t have to worry about your drive filling up with invalid data, but it doesn’t skirt the bigger issue: dynamic controllers see their performance improve with more free space.

My rule of thumb is to keep at least 20% free space on your drive, you can get by with less but performance tends to suffer. It doesn’t degrade by the same amount for all drives either. Some controllers are more opportunistic with free space (e.g. Intel), while others don’t seem to rely as much on free space for improved performance.

presumably this issue is even worse if you don't use Trim.

If you already use the SSD at full capacity you may have to do a secure erase to allow the firmware to treat the "unused" space as unused.

Warning following the instructions you might find after googling secure erase or sanitary erase might lead you to the point of having an unusable SSD. you are much better off making sure a new drive is never formatted to the full capacity so you can avoid this procedure. Given that disclaimer some quick links to read in addition to the Anand articles would be:

Nothing short of resetting the SSD to this "factory fresh" mode turns used space back into unused space. You can't just reformat the disk or resize the partition or use any traditional method you would have used on rotating disks in the past.

share|improve this answer
It really depends on how you define "unused". Does that mean unpartitioned or just not in use? – Sander Rijken May 11 '10 at 21:19
Sander. Unused in SSD terms means the SSD has no record of data ever being written to that bit of flash. Defining a 42GB partition on a 64GB flash drive that has never been used leaves unused space. Deleting that partition from the OS and creating a 25GB partition does NOT increase the amount of unused space. Only a "secure erase" procedure will reset the internal log in the SSD that allows all the flash be considered unused by the SSD firmware. – pplrppl May 13 '10 at 14:50
The worst case scenario here would be someone taking a brand new SSD and doing a "full format" with Windows 7 to a partition that is the full capacity of the SSD (which will attempt to write zeros to every sector during the format). At that point if they don't do a secure erase that SSD will behave like it's full performance wise even if you never fill even half the capacity with usable data. – pplrppl May 13 '10 at 15:16
Even worse would be for someone to do this then RMA the drive because it didn't benchmark well, the retailer selling it as an open box item, and someone paying near full price for a drive that doesn't perform well not knowing there is a procedure they could do to restore the drive back to proper performance. With luck they might get back most of the performance automatically over time as trim and garbage collection restructure the drive but it is also entirely possible for someone to use a SSD in a degraded state like that and it not recover. – pplrppl May 13 '10 at 15:19

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .