Jim Lux wrote:
That would be indicative of a terrible implementation, or the use of NOR
flash then. Particularly for SSDs, which almost certainly use NAND
flash, the write speed is very fast. It's the erase speed which is slow.
Some data from Toshiba I happen to have convenient gives the following
numbers:
NAND SLC 24 MB/s read, 8MB/s write, 2ms erase (smaller parts)
NAND MLC 18.6 MB/s read, 2.4 MB/s write, 2ms erase (bigger density)
NOR MLC 103 MB/s read, 0.47 MB/s, 900ms erase
I googled around and found:
http://www.storagesearch.com/easyco-flashperformance-art.pdf
In particular they mention for random writes: 47/sec, 24/sec, and 13/sec for
3 Flash SSDs they tested.
In the next chart they compare to disk performance and with 10% of the IOPS
being writes it was 1.5x worse than disk. 50% was 8x worse. 100% 16x worse.
Granted I don't know who funded the study, how current the SSDs are, but I've
heard plenty of complaints about SSD performance
There's also a difference in how you access it. NOR Flash is like
memory (address and data lines), NAND flash is more like a I/O device..
you tell where to start and you get streams of data in or out. The
Flash used for a BIOS would be NOR flash, for this reason.
Alas the paper didn't mention NAND or NOR (at least that I saw).
Sure, 2 MB/s isn't as fast as SDRAM, and even slower than some hard disk
drives, but don't forget there's zero rotational latency and zero seek
time, so overall, a flash based drive will probably be faster than a
spinning platter drive (aside from the much lower power, etc.) If you
were streaming video to the "disk", then you need to start thinking
magnetic media, rather than flash.
I can't explain it, but it seems either there's some biased reporting or some
of the SSDs out there are terrible at writes. Transcend, Samsun, and Sandisk
don't sound like terrible units to benchmark.
Seems common for the SSDs to be 2-10 times slower at random writes and
1/3rd
of the bandwidth (read or write) of common cheap SATA desktop disks.
That's probably about right.
I recently bought a cheap single platter WD drive for $70 or so, if I read
from the device directly (2MB at a time):
http://cse.ucdavis.edu/bill/wd320-2MB-points.png
Average 89MB/sec for reads. Granted random will be dominated by seek time.
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