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Add sections on vfs.write_behind and vfs.hirunningspace. Also note that

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this has already been comitted to stable (the MFC was really through
this commit, not the previous commit).
This commit is contained in:
Matthew Dillon 2002-06-25 02:47:55 +00:00
parent b895742d96
commit de795ba0f4
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=98792
1 changed files with 21 additions and 1 deletions
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22
share/man/man7/tuning.7
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@ -414,7 +414,27 @@ Such services can include web caches, large mail systems, and news systems.
Turning on this option will generally not reduce performance even with the
wasted memory but you should experiment to find out.
.Pp
There are various buffer-cache and VM page cache related sysctls.
The
.Va vfs.write_behind
sysctl defaults to 1 (on). This tells the filesystem to issue media
writes as full clusters are collected, which typically occurs when writing
large sequential files. The idea is to avoid saturating the buffer
cache with dirty buffers when it would not benefit I/O performance. However,
this may stall processes and under certain circumstances you may wish to turn
it off.
.Pp
The
.Va vfs.hirunningspace
sysctl determines how much outstanding write I/O may be queued to
disk controllers system wide at any given instance. The default is
usually sufficient but on machines with lots of disks you may want to bump
it up to four or five megabytes. Note that setting too high a value
(exceeding the buffer cache's write threshold) can lead to extremely
bad clustering performance. Do not set this value arbitrarily high! Also,
higher write queueing values may add latency to reads occuring at the same
time.
.Pp
There are various other buffer-cache and VM page cache related sysctls.
We do not recommend modifying these values.
As of
.Fx 4.3 ,