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Here we make
a comparison among the physical disk, the SCSI Ramdisk and the Direct
IO Ramdisk with different disk test tools.
The test platform has the following configuration:
physical harddisk: HITACHI HTS541616J9SA00
(SATA, 160GB, 5400
RPM, cache size: 8MB, cache to host data rate: 150MB/s)
CPU：Intel(R) Core(TM)2 Duo CPU T7500 @ 2.20GHz, 2194 MHz
physical memory：2GB + 2GB，DDR2-667 SDRAM
OS：Microsoft Windows XP Professional (32bit) SP3
All the test disks use NTFS file system with default cluster size. All
the test
ramdisks are based on OS managed memory (Visible Memory).
1. Iometer Test Result
Iometer
is an I/O subsystem measurement and characterization tool and is widely
used within the storage industry. Here we use the latest version
2006.07.27 to do tests with the following setup:
Test data size: 150MB, Outstanding I/Os: 1, Transfer
Block size: 512B，4KB，16KB，32KB, 64KB and 128KB, Test items:
100% sequential Read, 100% sequential Write, 100% random read and 100%
random write.
 
The following graphs show the test results of physical harddisk, SCSI
ramdisk and Direct IO ramdisk.
Note:
100% Seq Read: 100% sequential read
100% Rnd Read: 100% random read
100% Seq Write: 100% sequential write
100% Rnd Write: 100% random write
Physical Hard Disk
SCSI Ramdisk
Direct IO Ramdisk
We are easy to see that ramdisks have much faster transfer rate than
physical hard disks, especially in the condition of random access. It is
quite easy for ramdisks to achieve over 1GB/s transfer rate.
Also
we see that our ramdisks have almost the same transfer rate during
random access as sequential access. Some other
ramdisk programs have not this feature. Their performance on random
access
is quite bad compared with sequential access.
And we find that Direct IO ramdisk has better performance than
SCSI ramdisk. This is because the Direct IO ramdisk access the data
using direct I/O method and consequently reducing a lot of
internal system process time. A disadvantage of Direct IO ramdisk is that it
cannot be recognized by OS as a physical hard disk, only as a logical
drive.
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