Disk usage

Providing enough free disk space is one of the most urgent task for every system administrator. Applications often behave strange when the disks runs out of free space and sometimes data gets corrupt if the application is badly written.

Under Unix the 2 tools to determine the usage of the disk are “du” (disk usage) and “df” (disk free). With “du” you can determine the amount of space a certain directory (with the files and subdirectories in it) consume. “df” shows on a global level the amount of used and free disk space per partition.

Given the case, that you give the same directory (which is also a mountpoint) to both “du” and “df” as parameters. This directory contains a full CQ application with content and versions. You will probably get different results. When we did it, “df” showed about 570 gigabyte used disk space, but “du” claimed, that only 480 gigabyte are used. We checked for hidden directories, open files, snapshots and other things, but the difference of about 90 gigabyte remained.

This phenomenon can be explained quite easy. “du” accumulates the size of files. So if a file has 120 bytes in size, it adds 120 to the sum. “df” behaves differently, it counts block-wise, which are the smallest allocation unit of a unix filesystem (today most blocks are 512 bytes large by default). Because the only one file per block is possible, the 120-byte file uses a full block, leaving 392 bytes unused in that block.

Usually this behaviour is not apparant, because the number of files is usually rather small (a few to some ten thousand) and they are large, so the unused parts are at max 1 percent of the whole file size. But if you have a CQ contentbus with several hundert thousands of files (content + versions) with a lot of small files, this part can grow to a level, where you’d ask your system administrator, where the storage is gone.

So dear system administrator, there’s no need to move your private backup off the server, just tell the business, that their unique content structure needs a lot of additional disk space. 🙂

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