Understanding Flash SSD Performance
Douglas Dumitru
CTO EasyCo LLC
August 16, 2007
DRAFT
Flash based Solid State Drives are quickly becoming popular in a wide variety of applications. Most people think of
these solid state devices as just another hard disk, but their performance characteristics can be very different.
To start this discussion off, lets see a table of drive performance parameters for a couple of hard disk drives and a
couple of Flash SSDs.
Drive Model
Description Seek Time
Track
to
Track
Average Full
Stroke
Latency Read XFR Rate
Write XFR Rate
Outer
Tracks
Inner
Tracks
Outer
Tracks
Inner
Tracks
Hard
Drives
Western Digital
WD7500AYYS
7200 RPM
3.5” SATA
0.6 ms 8.9 ms
12.0 ms 4.2 ms
85 MB/sec
60 MB/sec* 85 MB/sec
60 MB/sec*
Seagate
ST936751SS
15K RPM
2.5” SAS
0.2 ms 2.9 ms
5.0 ms* 2.0 ms
112 MB/sec 79 MB/sec
112 MB/sec 79 MB/sec
Flash
SSDs
Transcend
TS8GCF266
8GB 266x
CF Card
0.09ms
40 MB/sec
32 MB/sec
Samsung
MCAQE32G5APP
32G 2.5”
PATA
0.14ms
51 MB/sec
28 MB/sec
Sandisk
SATA5000
32G 2.5”
SATA
0.125ms
68 MB/sec
40 MB/sec
* Figure is an estimate
Of course there are other hard drives and Flash SSDs on the market, but this mix is a good starting point to work with.
Read Performance
The write performance of Flash SSDs is very different from the read performance. So we will talk about read and
write performance separately.
When comparing read performance between hard disks and Flash SSDs the specs are often misleading. Flash SSDs
have much better access times, but typically are slower in terms of transfer rates. This means that you have to
consider the block size when evaluating read performance. Here is a chart that shows the average read performance
for these five drives at varying block sizes:
4
8
16
32
65
128
256
512
1024
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
45.00
50.00
55.00
60.00
65.00
70.00
Drive Read Performance
WD 7200 RPM
Seagate 15K RPM
Transcend 8G CF
Samsung 32G PATA
SanDisk 32G SATA
Transfer Size (KB)
Transfer Rate (MB/sec)
This is actually a very telling char