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Describe vfs.read_max and update vfs.hirunningspace documentation. While

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there, update nits.

Considered viewed by:	jeff, phk, rwatson
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Ivan Voras 2010-08-12 21:21:50 +00:00
parent 4be3feb212
commit c491a612fd
1 changed files with 27 additions and 9 deletions
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share/man/man7/tuning.7
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@ -23,7 +23,7 @@
.\"
.\" $FreeBSD$
.\"
.Dd January 23, 2009
.Dd August 11, 2010
.Dt TUNING 7
.Os
.Sh NAME
@ -470,7 +470,7 @@ sysctl defaults to 1 (on).
This parameter controls how directories are cached
by the system.
Most directories are small and use but a single fragment
(typically 1K) in the file system and even less (typically 512 bytes) in
(typically 2K) in the file system and even less (typically 512 bytes) in
the buffer cache.
However, when operating in the default mode the buffer
cache will only cache a fixed number of directories even if you have a huge
@ -504,17 +504,35 @@ it off.
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.
disk controllers system-wide at any given time.
It is used by the UFS file system.
The default is self-tuned and
usually sufficient but on machines with advanced controllers and lots
of disks this may be tuned up to match what the controllers buffer.
Configuring this setting to match tagged queuing capabilities of
controllers or drives with average IO size used in production works
best (for example: 16 MiB will use 128 tags with IO requests of 128 KiB).
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 occurring at the same
time.
Higher write queueing values may also add latency to reads occurring at
the same time.
.Pp
The
.Va vfs.read_max
sysctl governs VFS read-ahead and is expressed as the number of blocks
to pre-read if the heuristics algorithm decides that the reads are
issued sequentially.
It is used by the UFS, ext2fs and msdosfs file systems.
With the default UFS block size of 16 KiB, a setting of 32 will allow
speculatively reading up to 512 KiB.
This setting may be increased to get around disk I/O latencies, especially
where these latencies are large such as in virtual machine emulated
environments.
It may be tuned down in specific cases where the I/O load is such that
read-ahead adversely affects performance or where system memory is really
low.
.Pp
There are various other buffer-cache and VM page cache related sysctls.
We do not recommend modifying these values.