Using tmpfs to speed up "disk" access

August 01, 2011

Overview

If you have an application which performs a lot disk reads/writes and a modest amount of data, you don't need to keep. e.g. a unit tests, application builds. Using tmpfs may be the answer speeding up the process.

In this article I compare using a tmpfs, an SSD, a hard drive and an NFS drive.

Comparing creating a file and deleting it

Small files (256 bytes) are created repeatedly and an average time taken. These files are deleted in a second test.

File system type	machine	file creation	file deletion
tmpfs	fast	7.2 us	3.9 us
tmpfs	older	19.9 us	8.9 us
SSD ext4	fast	22.9 us	9.1 us
7200 RPM ext4	older	42.9 us	24.1 us
NFS	-	11,238 us	5,294 us

The "fast" machine was a 3.8 GHz i7 with 1600 MHz memory.

The "older" machine was a 2.5 GHz AMD 4800 with 533 MHz memory.

The NFS drive is a QNAP TS-210 with mirrored 7,200 RPM drives over a home plug network.

The write cache for the 7,200 RPM disk is making a big difference. Without the disk cache the performance would closer to that of the NFS file system.

In each case, the tmpfs was limited to 80% of the total main memory. Using a larger tmpfs will impact performance.

The Code

The core part of the test looks like this.

byte[] bytes = new byte[256];
long start = System.nanoTime();
for (int i = 0; i < files; i++) {
    File file = new File(dir, Integer.toString(i));
    FileOutputStream fos = new FileOutputStream(file);
    fos.write(bytes);
    fos.close();
}
long time = System.nanoTime() - start;
System.out.printf("Took an average of %.1f us to write to LOCAL filesystem%n", time/1e3/files);

OpenCloseFilesTest.java

Conclusion

Using a tmpfs can be 2-3x faster than a local disk with write cache. When you have working data you don't need to keep it can speed up your process.

Vanilla Java