SUMMARY: what is "fsflush"

From: Alan McKay (
Date: Thu Jan 04 1996 - 11:17:34 CST

Here are the answers I got on my question about what is "fsflush".
There were lots of requests for a summary, so here it is.

Note the last one has a useful URL to Sunworld Online.

Thanks a lot everyone!

My original message follows :

> Can anyone tell me anything about "fsflush"? I see it running on
> our Solaris 2.x systems, but can't find a manpage for it, nor can I
> even find the executable with the "find" command.



>From Wed Jan 3 17:34:45 1996

It is the moral equivalent of "update" in 4.x. fsflush is the SVR4
version - it is a system process, which means the kernel sets it up,
so there is not executable. The main difference is that fsflush tries
to spread the load of pushing stuff to disk instead of having a great
big hit every 30 seconds.

                l & h,


From: falcon!auspex! (Eric M. Stone)


fsflush is the Solaris 2.x equivalent to 'update' the SunOS filesystem
update daemon. This process is responsible for flushing the in-memory
inode table to the physical disk once every 30 seconds. A common cause
of filesystem corruption is to interrupt the OS before it has had a
chance to update the disk table, therefore making filesystem pointers
look in the wrong place for the data blocks (or some similar type


From: Tony Jago <>

 fsflush is an in-kernel process that is responsable for scanning through
 the memory pages and detecting which ones needed flushing to disk. If
 you have a large about of memory this process can begin to take a lot of
 cpu time. There is a system variable that you can adjust to vary the
 rate that the fsflush checks pages.


From: Scott McClung <>

fsflush is analogous to the 'update' process in SunOS 4.1.x, which
calls sync() every 30 seconds, to help ensure file system consistency.
In Solaris 2.x, it's not an actual executable, but created by the
kernel at boot time, along with sched and pageout. SunOS 4.1.x does
similar things to create swapper and pagedaemon in it's process table,
I think. Same idea, anyway.

Hope this helps... Later.


From: (Hal Stern - Distinguished Systems Engineer)

fsflush in solaris is what the "update" daemon was in sunos.
it is the daemon that flushes dirty filesystem pages out to disk
every 30 seconds. fsflush is governed by 2 tunables -- tune_t_fsflushr
and autoup. the flushrate (the first one) is how often the daemon runs;
the second one is the maximum age of a page (in seconds). by default,
they're both set to 30. you can make the flushrate smaller and increase
the maximum age to "spread out" the flush load, which is nice if you are
doing a ton of filesystem writes and don't want to or can't handle
huge spikes of disk activity. in that case,try setting autoup to 120
and the flushrate to 20 -- the daemon will run every 20 seconds, flushing
1/6 of memory until it has covered all dirty pages in 120 seconds.



It's a SVR4-ism. That's the program that "flushes" all cached data
from memory to disk so in case you get a computer crash, you have your
data saved in the disk. The default is one flush / minute. You can
change it via kernel parameter (don't know off hand). Basically, the
less frequent the flushing, the faster your computer is but you risk losing your
data more. More flushing, slower computer (more CPU cycles for flushing) but
less risk losing data.



From: (Michael J. Shon {*Prof Services}

fsflush gradually flushes "dirty" disk buffers out from the cache.
It does the "sync" that "update" did, but in a smoother manner.
Yes, it does use a lot of CPU searching the kernel page tables
for filesystem cache blocks. That can be tuned somewhat, but is not
normally necessary.

    Michael Shon (716) 385-5065 michael.shon@East.Sun.COM


From: "Rasana P. Atreya" <>

>From the Unix man pages on the net (at

fsflush -- Flushed by the sync
Every 30 seconds all the pages in the system are examined in physical page
order to see which ones contain modified data and are attached to a vnode.
The details differ between SunOS 4 and Solaris 2, but essentially any
modified pages will be written back to the filesystem, and the pages will be
marked as clean.



		5 out of 6 macho men prefer Russian roulette

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