So this seemed insane to me. SSD stalking has become a bit of an obsession of mine, and the thought that an SD card could beat out an HDD was hard enough; that it could beat out an SSD bordered on blasphemy.
But it has proved to be plausible. A gloss for those in the same boat as I was:
I just did a "hdparm -tT /dev/sda" on my bluelight-special 5400 rpm laptop drive and it is getting 3,100 MB/sec cached reads. The absolute fastest reads I see reported with the Sandisk Ultra cards on Amazon is 45 MB/s, with a few 30 MB/s reads, and mostly 20 MB/s reads.
Something doesn't add up here.
But the thing that really matters for bootup time, application load time, and application responsiveness time is 4k random reads. And 4k random reads for my bluelight-special 5400 rpm drive are:
4K random reads: 494 KB/s (KB!!!)[0]
According to the number one google result for "4k random read sandisk ultra"[1]:
4k random speeds: 4,499 KB/s
There are definitely some older SSDs that come in at around 2,350 KB/s (6,000 iops) for their 4k random reads.
So moral of the story: just buy a new SSD already with 4k random reads of 250,000+ KB/s.
Your harddrive is doing 3,100 MB/s? That's just over 3 GB/s, which I really have a hard time believing. Or is that a European comma, i.e. it's really doing 3.1 MB/s? Quite confusing. :/
Nope, American comma. Now that you mention it, it seems high to me as well. I just figured out why:
That is cached or buffered reads.[0] The buffer appears to be a small amount of memory that sits on the HDD. In addition to housekeeping, it seems mostly useful for read-ahead, which seems like it helps a little bit when you are accessing the same part of the HDD repeatedly (i.e., usually). On my HDD it is 8 GB.
RAM is faster than cache though. My RAM has a peak transfer rate of 10666 MB/s (DDR3 1333). Stuff sits around in your RAM after being loaded until something else needs the space. So buffers help with initial loads from the HDD. But after that, RAM is king.
For sequential reads/writes to my actual disk, the speeds are 15,000 KB/s to 50,000 KB/s (faster for larger files).
So yeah. SSD still the way to go for any form-factor that can handle it. But it's nice to know that SD cards have gotten to the point where they rival and sometimes beat HDDs in terms of real-world performance. The range of possible applications for the Pi and its ilk just expanded in my mind.
But it has proved to be plausible. A gloss for those in the same boat as I was:
I just did a "hdparm -tT /dev/sda" on my bluelight-special 5400 rpm laptop drive and it is getting 3,100 MB/sec cached reads. The absolute fastest reads I see reported with the Sandisk Ultra cards on Amazon is 45 MB/s, with a few 30 MB/s reads, and mostly 20 MB/s reads.
Something doesn't add up here.
But the thing that really matters for bootup time, application load time, and application responsiveness time is 4k random reads. And 4k random reads for my bluelight-special 5400 rpm drive are:
4K random reads: 494 KB/s (KB!!!)[0]
According to the number one google result for "4k random read sandisk ultra"[1]:
4k random speeds: 4,499 KB/s
There are definitely some older SSDs that come in at around 2,350 KB/s (6,000 iops) for their 4k random reads.
So moral of the story: just buy a new SSD already with 4k random reads of 250,000+ KB/s.
[0] Using iozone. command:
[1] http://www.raspberrypi.org/phpBB3/viewtopic.php?f=63&t=4...