Our testing indicates that the Vertex 4 128 GB drive's write performance is negatively impacted if sustained write activity extends beyond 50% of its free capacity. When this occurs, sequential and 4 KB random writes above a queue depth of one take a notable hit. When sustained writes extend to less than 50% of available capacity, however, performance is not affected. Therefore, the likelihood that you'd see the Vertex 4's potential suffer depends on the amount of free space you leave and the size of file transfers that occur within that free space.
We presented our findings to OCZ, but the company declined to comment. The inner workings of SSDs are not something that vendors like to dish on. However, OCZ has stated on its forum that the Vertex 4 employs two modes of operation: “performance mode” and “storage mode,” and that the drive is behaving as intended.
Update (6/27/2012): OCZ provided us with an official statement to clarify the behavior if its Vertex 4 armed with the latest firmware. We have not yet confirmed how long it takes the drive to transition back and forth from performance to storage mode and vice versa:
“Prior to the development of firmware 1.4 OCZ has been gathering usage data patterns for quite some time and between selected test case customers and internal SSD usage there are several trends that have repeatedly surfaced that we wanted to address directly with this latest update. One of these is that SSDs, more often than not, have large percentages of unused space. Examples include users leveraging SSDs as boot drives, for their hot data or simply for their more performance oriented applications. In the 1.4 firmware release OCZ leveraged these findings and optimized our garbage collection to provide a significant performance boost to users that fall into this category. Effectively what this means is that drives that are less than half full will enjoy further optimized performance and after crossing more than half full the garbage collection algorithm will re-optimize the drive for maximum efficiency based on a larger data footprint. During this transition there may be a small latency hit, but this is a onetime event, and overall performance quickly improves as the drive is now optimized for the larger amount of storage. OCZ feels that this firmware optimization further enhances the overall SSD experience for our customer base.”
From our observations on a partitioned drive, “storage mode” is encountered when sustained write activity exceeds 50% of the available free space. We can only speculate as to why write performance drops. We suspect that the Vertex 4 uses a very aggressive form of garbage collection that needs available capacity to work effectively, although that only partially explains some of our observations. The only way to avoid “storage mode” is to partition the drive to enable 50% over-provisioning.
While we haven't yet tested the 256 GB Vertex 4, we are led to believe that it doesn't take the same performance hit (though that might be due to the fact that a 256 GB drive is more likely to have more free space than a 128 GB drive). We also believe this issue is related to the customized firmware that OCZ uses and is unrelated to the Marvell controller within.