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Comments and suggestions concerning this book should be mailed to gmourani@videotron.ca © Copyright 1999-2000 Gerhard Mourani and Open Network Architecture ® 69 2.    The “bdflush” parameters The bdflush file is closely related to the operation of the virtual memory (VM) subsystem of the Linux kernel and has a little influence on disk usage. This file (/proc/sys/vm/bdflush) controls the operation of the bdflush kernel daemon. We generally tune this file to improve file system performance. Be changing some values from the default as show bellow, the system seems more responsive, e.g. it waits a little more to write to disk and thus avoids some disk access contention. · To change the values of bdflush, type the following command on your terminal: [root@deep /]# echo "100 1200 128 512 15 5000 500 1884 2">/proc/sys/vm/bdflush   In our example above, according to the“/usr/src/linux/Documentation/sysctl/vm.txt” file, the first parameter (100 %) governs the maximum number of dirty buffers in the buffer cache. Dirty means that the contents of the buffer still have to be written to disk (as opposed to a clean buffer, which can just be forgotten about). Setting this to a high value means that Linux can delay disk writes for a long time, but it also means that it will have to do a lot of I/O at once when memory becomes short. A low value will spread out disk I/O more evenly. The second parameter (1200) (ndirty) gives the maximum number of dirty buffers that bdflush can write to the disk in one time. A high value will mean delayed, bursty I/O, while a small value can lead to memory shortage when bdflush isn't woken up often enough... The third parameter (128) (nrefill) is the number of buffers that bdflush will add to the list of free buffers when refill_freelist() is called. It is necessary to allocate free buffers beforehand, since the buffers often are of a different size than memory pages and some bookkeeping needs to be done beforehand. The higher the number, the more memory will be wasted and the less often refill_freelist() will need to run. When refill_freelist() (512) comes across more than nref_dirt dirty buffers, it will wake up bdflush. Finally, the age_buffer (50*HZ) and age_super parameters (5*HZ) govern the maximum time Linux waits before writing out a dirty buffer to disk. The value is expressed in jiffies (clockticks), the number of jiffies per second is 100. Age_buffer is the maximum age for data blocks, while age_super is for file system metadata. The fifth (15) and last two parameters (1884 and 2) are unused by the system so we don’t need to change the default ones. You may add the above commands to the “/etc/rc.d/rc.local” script file and you’ll not have to type it again the next time if you reboot your system. Look at “/usr/src/linux/Documentation/sysctl/vm.txt” for more information on how to improve kernel parameters related to virtual memory. 3.    The “buffermem” parameters The buffermem file is also closely related to the operation of the virtual memory (VM) subsystem of the Linux kernel. The value in this file “/proc/sys/vm/buffermem” controls how much memory should be used for buffer memory (in percentage). It is important to note that the percentage is calculated as a percentage of total system memory. · To change the values of buffermem, type the following command on your terminal: [root@deep /]# echo "80 10 60" >/proc/sys/vm/buffermem In our example above, according to the“/usr/src/linux/Documentation/sysctl/vm.txt” file, the first parameter (80 %) means to use a minimum of 80 percent of memory for the buffer cache; the minimum percentage of memory that should be spent on buffer memory.