On 04/14/2015 at 12:55 PM, Bob Proulx wrote: > Petter Adsen wrote: > >> Can someone please enlighten me as to why the entry for this >> directory is so large, even though it is empty? Since it's >> apparently obvious to everyone else, I would very much like to know >> :) > > In the old days directories were simply an array of fixed sized > integers and fixed sized names. The fixed maximum size of the name > was 14 characters. The fixed size integer was the inode address. > That it was fixed sized allowed directories to be read into an array > of structures very efficiently using readdir(2) system call. > Individual entries were written by lseek(2)'ing to the proper slot > and writing just that fixed size entry leaving the rest of the > (special directory) file untouched. Directories were an array of > file entries. > > That strategy was in use for many years in Unix systems. But it has > unpleasant limitations. Short fixed sized file names 14 characters > or less were one. The linear nature of the array data structure was > another. The array influenced things considerably. When a file was > deleted the entree was simply zero'd out. lseek() to the proper > entry and zero it out. That meant that directories could grow in > size but never shrink. If a directory became full it was easy to > extend it by writing the array longer. But if an early entry in the > array was deleted the system would zero it out rather than move each > and every entry in the file system down a slot. (I always wondered > why they didn't simply take the *last* entry and move it down to the > deleted entry and simply keep the array always compacted. I wonder. > But they didn't do it that way.) I'd guess that it was in order to avoid having the "internal order" of files in a directory - which can affect the order in which they are processed, depending on how you get your directory listing - be changed by a seemingly unrelated operation. Thanks for the lengthy explanation and historical background. By the time I got started, apparently the 14-character limit was already far enough in the background that it never made it into the documentation that I read... -- The Wanderer The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man. -- George Bernard Shaw
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