Tempted by SSD Storage
Earlier this year, I finally upgraded the storage on my PC. Since 2009, it had been running with the same six 500 GB hard drives, configured as RAID. After experiencing ever slower start-up times, and having used a number of machines which have SSD storage, I decided it was time for a change.
My motherboard only supports SATA II, which has a 3.0 Gbps transfer rate, so I knew my interface might limit the performance of high-end SSDs. Therefore I went for higher-capacity drives at the slower end of the scale, thinking that anything would be an improvement over old-fashioned spinning disks.
SanDisk SSD Plus
I purchased two SanDisk SSD Plus 960 GB drives, which cost £396 in April, and mirrored them using RAID-1. Once everything was installed (which took a while, thanks to TrueCrypt preventing cloning), my PC started up about ten times faster!
Over the weeks that followed, I noticed that the machine sometimes seemed rather slow and unresponsive for several seconds at a time, especially when using my new document scanner, and when saving large files.
Suspecting the SanDisk SSDs, I decided to investigate further. I used Crystal Disk Info, and it sometimes showed very low write speeds — yet at other times, the speeds were more reasonable (though still nothing spectacular).
An online search suggested that for best performance, I should make sure my BIOS is set to use AHCI mode, rather than IDE mode. However, that option did not seem to be available with RAID, so I split my RAID-1 and tried running with a single SSD. Making that change finally allowed me to try AHCI mode, but unfortunately my PC crashed early on in the start-up process, so I had to revert back to IDE mode.
Samsung 860 EVO SSD
My next step was to take a chance and buy a faster SSD. I upgraded to a Samsung 860 EVO. I only got a 500 GB drive (£133 from a local shop — I didn’t want to wait), because in recent months, I’ve moved most of my files to my server. (In the past, I’d only used the server for photos and music.) Therefore, the smaller capacity is ample.
I used Macrium Reflect to clone my SanDisk drive to the new Samsung. Once I was running from the Samsung, the slow write speeds — and the regular slow-downs — went away.
Comparing SSD Write Speeds
I could have left it there, but I was curious as to what was going on with the SanDisk SSD Plus. First, I tried copying files between the Samsung and SanDisk drives. It was always fast copying to the Samsung, but slow copying to the SanDisk. I even tried cloning my new Samsung drive back to the SanDisk, and it took over 19 hours! (It had only taken about 2 hours to clone from the SanDisk to the Samsung.)
I wanted to know more about the SanDisk’s slow performance, so I wrote some code using C++ Builder to perform and monitor sustained writes. I ran the tests on both brands of SSD.
The results show that the Samsung performs well, for pretty-much all of the time. In contrast, the SanDisk SSD Plus performs much more slowly in general — and extremely slowly every so often.
Write speeds using both drives are extremely fast for the first 1000 MB or so, peaking at 2173 MB/s for the Samsung, and (strangely) even higher with the SanDisk at 3225 MB/s. I assume the data is just being cached, either by Windows and/or within the SSD. Therefore, when calculating the averages below, and in all but the first two graphs shown later, I’ve excluded data from that initial stage, to focus on sustained write performance instead.
Average Write Speeds
Samsung EVO 860: 217.3 MB/s
SanDisk SSD Plus: 27.0 MB/s
(Derived from six tests per drive. Note that encryption was used with the Samsung, which may slow it down a little. No encryption was used with the SanDisk. Both drives were connected to a SATA II 3.0 Gbps motherboard, which may have limited peak performance at times, and therefore reduced the above averages.)
The average figures don’t tell the whole story, so I’ve produced some graphs to give a better insight into the differences.
The following graph shows how long it takes to write each of a series of 100 MB files. This was performed continuously 79 times for one drive, then the other. The tall spikes show that writes to the SanDisk frequently stall — presumably this is due to wear-levelling taking place.
In another test, 5200 MB was written to each disk. This was repeated five times (after a rest period). The following graph shows the Samsung reaches the target size in a much faster and steadier fashion — the SanDisk takes much longer, varies a lot between runs, and sometimes pauses.
The graph below gives a closer look at a typical test using the SanDisk, which shows how the write speed fluctuates significantly — there are even moments when it drops to zero for up to 1.8 seconds (e.g. just before 90 seconds).
The graph below gives a closer look at a typical test using the Samsung drive. This also pauses from time to time, but never for more than 0.3 seconds. Note the different scale on the Y axis compared to the SanDisk — the Samsung is much faster.
The SanDisk has quite a wide spread of write speeds, as shown by the following histogram. Quite a significant percentage of writes are at lower speeds, which is something that you really notice when using a PC equipped with such a drive.
The Samsung drive performs most of its writes at around the same speed, as shown in the final histogram below.
It’s possible that a more modern motherboard might get better performance out of the SanDisk, but I think it would be unlikely to make a massive difference.
I would never recommend getting cheaper, slower SSDs for the main drive in any computer. Switching to the Samsung EVO 860 SSD has transformed the way my PC feels.
However, for an application that does not involve frequent writing to the drives, such as storing and serving a music collection, the cheaper SanDisk SSD Plus might still be a reasonable choice, because I have no complaints about its read performance.
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