MAN page from Old RedHat 5.X bdflush-1.5-8.i386.rpm

bdflush

Section: Maintenance Commands (8)
Updated: Aug 1994
Index

NAME

bdflush - kernel daemon to flush dirty buffers back to disk.

SYNOPSIS

bdflush [opt]

DESCRIPTION

bdflushis used to start the kernel daemon to flush dirty buffers back to disk. Theactual dirty work is in a kernel function, and bdflush actually forks a newprocess which then calls the kernel function that will never return.

bdflushactually forks a second daemon as well, and this one acts like a moretraditional update process, except that buffers are not considered ready to bewritten until they have aged a bit. The clock starts when the buffer isbrelsed with the dirty bit set, and the buffer will not be written back untilan interval passes. The interval is different for data buffers and formetadata buffers (like directories, bitmaps, indirect blocks, etc), and thecurrent settings are displayed by bdflush when you run it with arguments onthe command line. The defaults are 30 seconds for data buffers and 5 secondsfor metadata buffers.

The two daemons are normally started in /etc/rc with one command:

/sbin/update

Note that you need to have both daemons running because each one serves adifferent purpose. Note also that this command should be runbefore any major I/O is performed. In particular, update should becalled before any filesystems are fsck'ed or mounted read-write.

When bdflush is called by a user without superuser priveledges, itcalls flush() and sync() and then exits. Mustn't have 20 updatedaemons running at a time...

COMMAND-LINE OPTIONS

-d: Display kernel parameters. Using this option prevents the daemon from starting.
-h: Print the usage message (help).
-s: If bdflush defaults to the old update behavior, call sync this often.( In seconds ) Default:30.
-f: Call flush this often. ( In seconds ) Default:5.
-0: Max fraction of LRU list to examine for dirty blocks.
-1: Max number of dirty blocks to write each time bdflush activated
-2: Num of clean buffers to be loaded onto free list by refill_freelist
-3: Dirty block threshold for activating bdflush in refill_freelist
-4: Percentage of cache to scan for free clusters
-5: Time for data buffers to age before flushing
-6: Time for non-data (dir, bitmap, etc) buffers to age before flushing
-7: Time buffer cache load average constant
-8: LAV ratio (used to determine threshold for buffer fratricide)

AUTHOR

bdflushwas written as a quick hack by Eric Youngdale <ericyAATTgnu.ai.mit.edu>. The maingoal was to improve kernel performance by making the flushing of dirty buffersmore intelligent, and add support for buffer clustering. Feel free to improveit. Various other hacks by Phil Bostley <bostleyAATTcs.colorado.edu> andDaniel Quinlan <quinlanAATTyggdrasil.com>.

BUGS

If there are, they are most likely in the kernel code.

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