OCZ Agility 4 (256GB) Reviewby Kristian Vättö on September 1, 2012 1:00 PM EST
Performance Over Time & TRIM
For starters, I ran HD Tach on a secure erased drive to get the baseline performance:
What you are seeing in the above graph is a new feature in the 1.5 firmware called performance mode. At first, write performance is great at nearly 400MB/s but after about 25% of the drive has been filled, write speed drops to ~200MB/s. Once ~80% of the drive has been filled, write speed drops to around 75MB/s. At smaller drive utilization points, OCZ takes advantage of having a lot of unused NAND and through some proprietary firmware magic it enables lower write latencies. As you fill the drive, OCZ's firmware has to reorganize internal pages and thus you see tangible performance drops as you pass certain capacity points.
Next I secure erased the drive, filled it with compressible data and tortured it with 4KB random writes (QD=32, 100% LBA space) for 20 minutes:
The behavior we are seeing here is similar to Vertex 4. Right after torture performance is pretty bad but garbage collection is doing its job as soon as you have written some sequential data to the drive.
I secure erase the drive and reran our torture test but instead of 20 minutes of treatment, I tortured the drive for 60 minutes:
Write speed drops to as low as 20MB/s for the earliest LBAs. With most drives the worst case write speed is around 40-50MB/s, although asynchronous NAND probably has some impact here. Fortunately write speed is again restored when writing sequential data to the drive.
After running HD Tach, I let the drive idle for 30 minutes and reran HD Tach:
Everest 2 isn't very aggressive when it comes to idle garbage collection. Write speed already restored with first HD Tach pass and it stays at ~210MB/s. Remember that the drive is full of data and hence not running in performance mode; ~200MB/s write speed is normal when the drive is running in storage mode. Interestingly enough, read speed also degraded when tortured, and restored with idle time. This is not typical for non-SandForce SSDs, we'll have to do some more digging regarding this.
Finally I TRIM'ed the drive:
And TRIM works as it should. Formatting the drive sends a command to delete data in all user accessible LBAs, hence the drive is running in performance mode again for the first 25%.