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Linux Journal: Tweaking Tux, Part 2

Sep 21, 2000, 23:52 (0 Talkback[s])
(Other stories by Marcel Gagné)

"What I gave you last time were all network tweaks. This time around, I want to show you a few file system tricks. In past lives (working with other UNIXes), the systems I administered ran complex databases, often with hundreds of users. I'm fond of the following tweaks because they represent parameters that require a kernel rebuild if you find yourself starting to run low. You made your best guess, but invariably, it would be kernel rebuild time soon enough. With Linux, these parameters are simple /proc tweaks. If you are running a busy database system with a large number of users, this is one you might run into. The "file-max" parameter defines the maximum number of open files on your system at any given time. For most, the default "4096" is plenty. For a busier system, you might want to push that limit up somewhat. As an example, let's double that number."

    echo "8192" > /proc/sys/fs/file-max

"If you get errors stating that you are running out of file handles, it's definitely time to change that number, but don't wait for the users to start ringing. Without waiting for errors, you can take a look under the hood and see when this limit is approaching. (Preventative maintenance. What a concept.) If you do a cat on /proc/sys/fs/file-nr, you will get three numbers. The third will be your file-max. The first and second are the number of allocated file handles and the number of actual used file handles. Why the two numbers? When the Linux kernel allocates a file handle, it does not release it. If you do increase the file-max value, then you should also increase inode-max as well. Considering that each open file requires an inode for stdin, stdout (and possibly, a network socket) this needs to be somewhat higher than your file-max. Take your file-max value, triple it and write it back out to inode-max."

    echo "24576" > /proc/sys/fs/inode-max

"Busy web server? News server? Here's another tweak for your files, and this one has nothing to do with /proc. One of the options for the mount command is "noatime". In other words, do not (don't you even think about it) update the access time on visited files. Each time a file is read, the access time is updated which can yield useful information about file usage (with the find command, for instance). Odds are, you may not need that information. In the case of a web server getting a few thousand hits a day (an hour?), this little change can make a difference. Historically, this option was a suggestion for directories on news servers. Today, we are usually talking web servers. This is an environment where small files are accessed over and over again ( vs. a database environment which traditionally has comparatively few, large files)."

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