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When searching for a library file from a right folder, my application currently does couple of lstat's for the possible locationslocations. Now I'm wondering whether these lstat's (application is being a web service, thereby there can be hundred's of lstat's per second) cause always a an additional disk seek, or are those seeks cached?

System is Linux (2.6. kernel) with ext4 -based filesystem.

Part of PHP-CGI strace:

PHP-CGI strace:

lstat("/home/www/mydomain.com/inc/myLibrary/php/classes/myClass.php", 0x7fffa805a710) = -1 ENOENT (No such file or directory)
lstat("/usr/share/php/inc/myLibrary/php/classes/myClass.php", 0x7fffa805a710) = -1 ENOENT (No such file or directory)
getcwd("/home/www/mydomain.com/misc", 4096) = 32
lstat("/home/www/mydomain.com/misc/./inc/myLibrary/php/classes/myClass.php", 0x7fffa805a710) = -1 ENOENT (No such file or directory)
lstat("/home/www/mydomain.com/inc/inc/myLibrary/php/classes/myClass.php", 0x7fffa805a710) = -1 ENOENT (No such file or directory)
lstat("/home/www/mydomain.com/inc/myLibrary/php/classes/myClass.php", 0x7fffa805a660) = -1 ENOENT (No such file or directory)
lstat("/usr/share/php/inc/myLibrary/php/classes/myClass.php", 0x7fffa805a660) = -1 ENOENT (No such file or directory)
getcwd("/home/www/mydomain.com/misc", 4096) = 32
lstat("/home/www/mydomain.com/misc/./inc/myLibrary/php/classes/myClass.php", 0x7fffa805a660) = -1 ENOENT (No such file or directory)
lstat("/home/www/mydomain.com/inc/inc/myLibrary/php/classes/myClass.php", 0x7fffa805a660) = -1 ENOENT (No such file or directory)
getcwd("/home/www/mydomain.com/misc", 4096) = 32
lstat("/home/www/mydomain.com/misc/inc/myLibrary/php/classes/myClass.php", 0x7fffa805c7c0) = -1 ENOENT (No such file or directory)
lstat("/home/www/mydomain.com/misc/inc/myLibrary/php/classes", 0x7fffa805c600) = -1 ENOENT (No such file or directory)
lstat("/home/www/mydomain.com/misc/inc/myLibrary/php", 0x7fffa805c450) = -1 ENOENT (No such file or directory)
lstat("/home/www/mydomain.com/misc/inc/myLibrary", 0x7fffa805c2a0) = -1 ENOENT (No such file or directory)
lstat("/home/www/mydomain.com/misc/inc", 0x7fffa805c0f0) = -1 ENOENT (No such file or directory)
open("/home/www/mydomain.com/misc/inc/myLibrary/php/classes/myClass.php", O_RDONLY) = -1 ENOENT (No such file or directory)


/* FILE FOUND */

open("/home/www/mydomain.com/inc/classes/myClass.php", O_RDONLY) = 7
fstat(7, {st_mode=S_IFREG|0775, st_size=7734, ...}) = 0
fstat(7, {st_mode=S_IFREG|0775, st_size=7734, ...}) = 0
fstat(7, {st_mode=S_IFREG|0775, st_size=7734, ...}) = 0
fstat(7, {st_mode=S_IFREG|0775, st_size=7734, ...}) = 0
mmap(NULL, 7734, PROT_READ, MAP_SHARED, 7, 0) = 0x7f2225786000
stat("/home/www/mydomain.com/inc/classes/myClass.php", {st_mode=S_IFREG|0775, st_size=7734, ...}) = 0
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1 Answer 1

Somehow the OS needs the information, wether the file is there or not. Obviously it has to read in the folder list of all parent folders where you are looking for the file.

But don't worry too much. Once read in, these informations will remain in the caches, so it does not have to search on the disk every time - only if for some reason the cached informations have been dropped (maybe the OS needed this space in memory for more recent data) the OS will have to look on the disk again.

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1  
If you want to prove this to yourself, try this out. Restart the system, so the cache is cleared out. Then run a "find" command. Watch as your disk seeks. Once the command finishes, run the command again. They amount of disk activity will be noticeably lower. –  Nighthawk Oct 14 '11 at 11:40

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