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SSD 2011 roundup: Crucial M4, OCZ Vertex 3, Intel 510/320
by Marc Prieur
Published on June 23, 2011

Performance over time & TRIM
As we’ve already mentioned on several occasions on this website, an SSD’s performance levels can deteriorate over time. There are several reasons for this, the first being structural: a hard drive can read, write (unwritten space) or rewrite (space that has previously been written to) data by 4 KB packages. With flash memory you can only read, write or rewrite by 4 KB packages, 4 KB and … 512 KB (or even 8 KB, 8 KB and 2048 KB for a 25nm 8 GB flash chip).

When a space that is already occupied by a file has to be rewritten, there can be an impact on performance! Worse still, if the file has been deleted by the OS and the OS doesn’t support TRIM, the SSD then doesn’t know that the file has been deleted and acts as if it had to rewrite the data rather than write it. This problem is however resolved with TRIM as the OS then tells the SSD that the space is to be considered as unwritten.

This structural issue is accentuated by the existence of optimisations inside SSDs aimed at improving random write performance and write amplification. Simply put, when a request to write data randomly to an SSD is received, the SSD writes it sequentially to the flash memory, making sure that the addresses in its internal allocation table correspond to the addresses known by the OS (the LBAs) and the corresponding flash pages. For such a mechanism to be efficient, the blocks of flash memory do have to be available, which can be problematic without TRIM support.

What does all this translate to in practice? Testing the impact of wear on an SSD’s performance is no easy thing. We nevertheless carried out a few tests on the 120/128 GB versions of the Intel SSD 510, Intel SSD 320, OCZ Vertex 3 and Crucial M4.

The idea behind our test is simple. We put the SSDs in an extreme situation. We created a partition, leaving only 10 GB of space on the SSD that we then filled with video files. By leaving just a small amount of available flash on the SSD, this accelerated any performance deterioration.

We then carried out several tests for 3 minutes with IOMeter on the remaining 10 GB, using data that can’t be compressed a great deal:

- Random writes (= "New" graph)
- Sequential writes (= "Used" graph)
- 10 GB formatting using the TRIM command
- Sequential writes (= "TRIM" graph)
- Random writes (= "Used" graph)
- 10 GB formatting using the TRIM command
- Random writes (= "TRIM" graph)

Finally, we reset the SSD and, after putting it back in the same state, we carried out sequential writes to the SSD (= "New" graph)

Here are our results:

[ New ]  [ Used ]  [ Trim ]

Writing to a space previously used for sequential writes doesn’t have much of an impact on random writes, as you can see if you compare the differences between the ‘new’ and ‘used’ graphs. The impact of TRIM support can’t therefore be measured in this case. Note however that the Intel SSD 320 is least impacted, with peformance only dropping off at the end of the test.

In fact the SSD 320’s performance does drop after 210 seconds, when the SSD has already written 7.59 GB to the 10 GB partition (but write amplification has to be added to this). The dip in performance actually comes latest on the Vertex 3, after 180 seconds, by which time it has already been asked to write 13.5 GB to the 10 GB partition, but without helping it with TRIM! On the SSD 510 and Crucial M4, the dip in performance comes more quickly, after 60 and 90s, or respective writes of 3.9 and 6.4 GB. This is still a good level of performance.

[ New ]  [ Used ]  [ Trim ]

Things change with sequential writes. On a new SSD, the level of performance remains stable throughout the test, even though 30 to 45 GB of data is being written continuously. The only exception to the rule is the Vertex 3, whose performance takes a hit after 60s (or 9.4 GB written), dropping from 161 to 142 and then 132 MB/s.

This can be felt when writing sequentially to a space that was previously written with random writes, except on the SSD 320, which is hardly affected. Performance levels on the SSD 510, M4 and Vertex 3 then drop to 120, 83.1 and 48.3 MB/s respectively in the first 30 seconds! As you continue to write to the used SSDs however, performance gradually return to the sort of levels you’d expect and by the end of the test they’re all quite close to their initial performance.

Using TRIM allows the Crucial M4, Intel SSD 320 and 510 but not the Vertex 3 to kick in with their initial levels of performance from the start. The Vertex 3 stays at around 130 MB/s (against 160 MB/s when ‘new’) and even suffers a few other dips during the test. Remember however that we have put the SSDs in what are extreme conditions for the SandForce controller here as we used data that either wasn’t easily compressed or couldn’t be compressed at all, whether this was videos stored on the first partition or writes carried out with IOMeter.

In the end, the new SSDs do pretty well and even though we do advise you to use an OS that is TRIM compatible, it isn’t totally out of the question to use them when TRIM isn’t supported.

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