Diskfree() performance

chipperia · Joined: 25 Mar 2006 Posts: 13

I'm building a data logger on a 1 GB SD, and wondering about the best file structure (need to record about 10 kB of data every second or so).
I think that deeply nested directory should make file access easier and more faster, especially if each directory contains a limited number of files in order to keep its size within one sector.

For example, making different subdirectories for year, month, day, hour, minute, and at last a different data file for each second should grant file retriaval after accessing new sectors about 5 or 6 times (Y+M+D+H+M = 5); is that assumption correct?

Next step is to detect when the card is (almost) full, in order to delete automatically some of older files to make space for fresh data.
Is Diskfree() suitable for this use? My guess is it works scanning the FAT, which can take quite long especially for FAT32 volumes.

Any suggestion for a quick-and-dirty way for estimating disk occupation?

oe9vfj · Joined: 17 Jun 2004 Posts: 269 Location: Austria, Hard

You are right. DiskFree is scanning the FAT Sectors for free marked cluster entries.
On a FAT16 partition there is a maximum of appr. 250 FAT Sectors. On a FAT32 partition you have much more FAT-Sectors to scan.

With FAT16 you have in root-directory 'only' 512 directory entries. Please consider, that a long file name (not supported from AVR-DOS) occupies several directory entries. With a FAT16 on a 1GB partition you have a cluster-size of 16KB, so small files always occupies storage size of 16KB on the card.
In a sub-directory you have on FAT16 and FAT32 in AVR-DOS a limit of 65530 directory entries.
I hope this information helps to plan your application.