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I was wondering if having less than 5-10GB of free space on an SSD would affect performance. Is it specific to SSD or is it more of an operating system aspect?

For example, on an Intel X-25 G2 that has TRIM support (on Windows 7).

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Most SSDs will slow down as they fill up. SSDs tend to chose empty blocks to write in, since they can only modify a block by erasing and re-writing the entire block. Once all of these blocks are partially filled, then every write operation will require an erase and rewrite, meaning the SSD has to read the block into cache, modify the cache with the new data, then erase the block and write the cache. This comes out to one read and two programming operations for each block that needs to be modified. Also, though fragmentation has very little effect on SSDs, high fragmentation and the fact that most OSes are ignorant of SSDs can cause the OS to issue more writes than needed. The SSD's blocks are usually larger than the OS's allocation units, so if the SSD is not expecting the extra writes, it could end up repeating the same three-step process multiple times per block, thus slowing down writes even more. Even deleting files does not solve this issue, because the data remains there, and the SSD doesn't know that it is no longer used, and continues rewriting this deleted data.

The TRIM operation, which your drive supports, allows the OS to tell the drive to get rid of the deleted data. This allows the drive to reclaim empty blocks. However, the only operating systems that support TRIM are Windows 7, Windows Server 2008 R2, and Linux distributions using kernel 2.6.33 or later.

Since you have Windows 7, your system is using the TRIM command, and is able to reclaim the unused blocks. However, TRIM does not rearrange the data, so if the drive is close to full, high fragmentation can still cause data to partially occupy blocks, and limits TRIM's effectiveness. The best thing to do as far as I can tell is to free up as much disk space as you can. If you can free up enough space, then full blocks can be trimmed and your drive will get some speed back. Also, there are defragmentation programs that are geared towards SSDs, meaning that they will only defragment enough to trim a few extra blocks, but I don't know of any free ones. I think Diskeeper is supposed to do this, but it's expensive and I've heard very mixed reviews.

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    The FTL layer sits below the filesystem level. While it may look like the filesystem is fragmented, this does NOT mean that the FTL layer is fragmented. Those SSD "defrag" programs are worthless, they can't get access to the FTL layer, it is snake oil. Now, lack of free space does make SSD slowers, in that it has less room to do garbage collection, and wear leveling, & all that good stuff. SSD's don't automatically pick empty blocks per se, they need to take into account of wear leveling as well. Bottom line is, filesystem does NOT know anything about how it stores things on the FTL layer
    – Time Twin
    Dec 5, 2016 at 20:21
  • Filesystems not optimized for SSD fail to trim as much, which has the same effect as smaller free space.
    – Basilevs
    Jul 16, 2018 at 5:19
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In this excellent article at AnandTech, they explore the relationship between the spare area and the SSD performance. The bottom line is that all SSDs gain/keep performance when leaving some of their space unused. Sometimes there's even a difference between 25% free space and 50% free space.

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It depends on a specific SSD model, vendor and usage patterns.

In some situations an intensively used SSD may deliver a significantly degraded performance disregarding of the amount of used disk space requiring a low-level wipe to restore to a like-new performance level.

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  • For example, on an Intel X-25 G2 that has TRIM support (on Windows 7).
    – Gabe
    Jul 10, 2010 at 22:35
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I have heard that SSDs tend to slow down when they get nearly full, but I am guessing that it would need to have less than 5-10GB of free space left for you to feel anything like that. Maybe when you have less than 1GB left or something.

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Filesystems in general get slow when they get full. It's due to fragmentation. On an empty disk, you can be sure that you can write a file in one block. As the filesystem gets fuller, then the chances of being able to fit the file into a single contiguous block decreases, and you're having to do more and more seeks to find the data, converting what was originally a sequential access of data into a more like a random access of data, and thus slower.

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    This doesn't apply to SSDs because there are no moving parts, therefore random accesses add a negligible amount of access time. You might have to have a file jumping around with thousands of randomly placed fragments to have any noticeable impact, which would mostly be from the OS itself planning its reads and writes.
    – TuxRug
    Jul 11, 2010 at 2:12
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    There is still a difference between sequential reads and random reads. For example codinghorror.com/blog/2008/06/… gives 26MB/s for sequential read, and 6Mb/s for random read.
    – gorilla
    Jul 11, 2010 at 10:37
  • @TuxRug: Drives are grouped into erase-blocks of hundreds of Kbytes or more; once a page (representing 512 or 4096 bytes of "logical disk" data) is written, it can't be rewritten except by erasing all of the pages within its block. When a "logical disk" sector is overwritten, the drive allocates a formerly-blank page to hold its new contents and notes that the old page can be considered obsolete. Rather than erasing obsolete pages immediately, most drives will wait until they need the space and then look for a block where most or all of the pages are obsolete.
    – supercat
    Aug 1, 2016 at 16:39
  • @TuxRug: If the number of empty pages gets critically low, drive performance will proportional to the largest number of free or obsolete pages within any block [if there are 10 blocks each with 15 obsolete pages, then for the next 150 writes, a write/erase cycle will be needed every 15th. If there were three blocks each with 50 obsolete pages, then a write/erase cycle would only be needed on every 50th write.] Fragmentation makes it more likely that one will end up with lots of blocks that have only a few obsolete pages each.
    – supercat
    Aug 1, 2016 at 16:48
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Another important factor is if and how much over-provisioning is set by the SSD manufacturer, as explained here. Over-provisioning means that some space is reserved for background activities such as garbage collection, and not made available to the user. For example, an SSD advertised as 256 GB may actually be 282 GB if 10% over-provisioning is set.

If the SSD manufacturer has been generous with over-provisioning space, low disk space may not affect the performance greatly, e.g. even an SSD that appears 100% full may actually be only 85% full. But if you are able to actually fill your SSD close to 100%, that would cause a significant performance degradation.

In short, the amount of the effect and whether it will be noticeable is SSD specific. But as a general rule, SSDs perform best when they are mostly empty and their performance deteriorates as they fill, for reasons explained in other answers and in the link.

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