UNIX for Dummies Questions & Answers

Hi All,
I am working on a Solaris machine. When i use a particular software to generate some files, the log shows around 0 to 3 secs for generating each file. But the same when i see on the disk it shows double the time difference between two file generation.

For example if file A takes 0 secs and file B takes 2 secs and file C takes 2 secs, when i give a ls command the output shows that file A created at 12:00:00
B created at 12:00:08
C created at 12:00:14


Most files generated in 0 secs show 8 seconds and others show double the time.
What could be the problem


Thanks in advance

ls is showing the last time that the file was written. And I believe in the accuracy of the time it displays. If that does not jibe with the log produced by your "particular software", you should ask the author of that software to explain.

Doesnt *nix cache writes sometimes to speed up interactvity?

Yes it does, but it still accurately records when the program issued the write(). The data may not find it's way to disk until several secords later. But the inode will record when the write occurred. A record is not kept of when the data was actually synced to disk.

Okay, I new it was a long shot

"The data may not find it's way to disk until several secords later. "
What could be the reason for this???.

Unix has a buffer cache which is a very large chunk of main memory.

When you issue a disk read, unix sees if the data is already in the buffer cache, if so no i/o occurs, the data is just moved into your program's buffer. When you issue a write, the data is just moved into the buffer cache.

The system periodicly scans the buffer cache and flushes new data to disk. This is called "write-behind". The system also predicts data that you are likely to need and issues reads to get the data into the buffer cache before your program wants it. This is called "read-ahead".

Read-ahead and write-behind work so well that most disk i/o is reduced to just moving data in memory.