Continuous activity test
We recently began testing SSDs to find out how well they perform when they are in continuous use for 15 minutes. We did this with the 250 GB and 256 GB models of the 840 and 840 Pro.
For an SSD to be able to write, the data cells first have to be emptied. This can only happen per block, and a block consists of 128 pages and is therefore 512 kB.
SSDs therefore have to employ some smart tricks. When a number of pages of data have to be removed, the rest of the data first has to be copied to another block, after which the entire block can be emptied. In practice that means that SSD controllers save up writes as much as possible. When the SSD is idle, the controller activates garbage collection, which removes the data from the chips and combines the left-over data as much as possible in complete blocks. This ensures that as many blocks as possible are made available for write actions.
However, if the SSD is continuously in use, the garbage collector doesn't get the chance to do this. Eventually this will cause all blocks to be occupied, which will force the SSD to employ garbage collection between commands. This has a negative effect on performance.
To illustrate this impact on performance, we performed a 15-minute test on several SSDs with the IOMeter benchmark. The settings were as follows: 32 simultaneous instructions (queue-depth 32), with 50% reading and 50% writing. The instructions consists of 25% 4 kB, 25% 64 kB, 25% 128 kB and 25% 1 MB. IOmeter used a 160 GB test file.
In the chart you'll see that most SSDs suffer a slow-down after about seven minutes. With the OCZ Vector the performance hit comes much sooner, but the stable point is indeed higher than with the other SSDs. The Vector stabilises just above 300 MB/s. The Corsair Neutron GTX 240 GB with LAMD controller also does well here. The Samsung 840 Pro dips to 230 MB/s after seven minutes.
In real life it will rarely happen that your SSD will be in contual use for that long. However, the fuller your SSD is, the fewer empty blocks that are available, and the sooner the performance hit will occur.
The Samsung 840 Pro dips to its stead state after about seven minutes, arriving at 230 Mb/s. The 840 starts at 200 MB/s, and after 15 minutes goes down to below 100 MB/s.