tuning(7)

NAME

tuning - performance tuning under FreeBSD

SYSTEM SETUP - DISKLABEL, NEWFS, TUNEFS, SWAP

When using bsdlabel(8) or sysinstall(8) to lay out your file
systems on a hard disk it is important to remember that hard drives can
transfer data much more quickly from outer tracks than they can from inner
tracks. To take advantage of this you should try to pack your smaller
file systems and swap closer to the outer tracks, follow with the larger
file systems, and end with the largest file systems. It is also important
to size system standard file systems such that you will not be forced
to resize them later as you scale the machine up. I usually create, in or
der, a 128M root, 1G swap, 128M /var, 128M /var/tmp, 3G /usr, and use
any remaining space for /home.
You should typically size your swap space to approximately
2x main memory. If you do not have a lot of RAM, though, you will gen
erally want a lot more swap. It is not recommended that you configure any
less than 256M of swap on a system and you should keep in mind future
memory expansion when sizing the swap partition. The kernel's VM paging
algorithms are tuned to perform best when there is at least 2x swap
versus main memory. Configuring too little swap can lead to inefficiencies
in the VM page scanning code as well as create issues later on if you
add more memory to your machine. Finally, on larger systems with multi
ple SCSI disks (or multiple IDE disks operating on different controllers),
we strongly recommend that you configure swap on each drive. The swap
partitions on the drives should be approximately the same size. The ker
nel can handle arbitrary sizes but internal data structures scale to 4
times the largest swap partition. Keeping the swap partitions near the same
size will allow the kernel to optimally stripe swap space across the N
disks. Do not worry about overdoing it a little, swap space is the
saving grace of UNIX and even if you do not normally use much swap, it can
give you more time to recover from a runaway program before being forced
to reboot.
How you size your /var partition depends heavily on what you
intend to use the machine for. This partition is primarily used to
hold mailboxes, the print spool, and log files. Some people even make
/var/log its own partition (but except for extreme cases it is not worth the
waste of a partition ID). If your machine is intended to act as a mail
or print server, or you are running a heavily visited web server, you
should consider creating a much larger partition - perhaps a gig or
more. It is very easy to underestimate log file storage requirements.
Sizing /var/tmp depends on the kind of temporary file usage
you think you will need. 128M is the minimum we recommend. Also note
that sysinstall will create a /tmp directory. Dedicating a partition for
temporary file storage is important for two reasons: first, it reduces the
possibility of file system corruption in a crash, and second it reduces
the chance of a runaway process that fills up [/var]/tmp from blowing up
more critical subsystems (mail, logging, etc). Filling up [/var]/tmp is a
very common problem to have.
In the old days there were differences between /tmp and
/var/tmp, but the introduction of /var (and /var/tmp) led to massive confusion
by program writers so today programs haphazardly use one or the other
and thus no real distinction can be made between the two. So it makes
sense to have just one temporary directory and softlink to it from the
other tmp directory locations. However you handle /tmp, the one thing you
do not want to do is leave it sitting on the root partition where it
might cause root to fill up or possibly corrupt root in a crash/reboot situa
tion.
The /usr partition holds the bulk of the files required to
support the system and a subdirectory within it called /usr/local holds
the bulk of the files installed from the ports(7) hierarchy. If you do
not use ports all that much and do not intend to keep system source
(/usr/src) on the machine, you can get away with a 1 gigabyte /usr partition.
However, if you install a lot of ports (especially window managers and
Linux-emulated binaries), we recommend at least a 2 gigabyte /usr and if
you also intend to keep system source on the machine, we recommend a 3 giga
byte /usr. Do not underestimate the amount of space you will need in this
partition, it can creep up and surprise you!
The /home partition is typically used to hold user-specific
data. I usually size it to the remainder of the disk.
Why partition at all? Why not create one big / partition
and be done with it? Then I do not have to worry about undersizing
things! Well, there are several reasons this is not a good idea. First,
each partition has different operational characteristics and separating
them allows the file system to tune itself to those characteristics. For
example, the root and /usr partitions are read-mostly, with very little
writing, while a lot of reading and writing could occur in /var and
/var/tmp. By properly partitioning your system fragmentation introduced in
the smaller more heavily write-loaded partitions will not bleed over in
to the mostlyread partitions. Additionally, keeping the write-loaded
partitions closer to the edge of the disk (i.e., before the really big
partitions instead of after in the partition table) will increase I/O
performance in the partitions where you need it the most. Now it is true
that you might also need I/O performance in the larger partitions, but they
are so large that shifting them more towards the edge of the disk will
not lead to a significant performance improvement whereas moving /var to
the edge can have a huge impact. Finally, there are safety concerns.
Having a small neat root partition that is essentially read-only gives it a
greater chance of surviving a bad crash intact.
Properly partitioning your system also allows you to tune
newfs(8), and tunefs(8) parameters. Tuning newfs(8) requires more experi
ence but can lead to significant improvements in performance. There are
three parameters that are relatively safe to tune: blocksize,
bytes/i-node, and cylinders/group.
FreeBSD performs best when using 8K or 16K file system block
sizes. The default file system block size is 16K, which provides best
performance for most applications, with the exception of those that per
form random access on large files (such as database server software).
Such applications tend to perform better with a smaller block size, al
though modern disk characteristics are such that the performance gain from
using a smaller block size may not be worth consideration. Using a
block size larger than 16K can cause fragmentation of the buffer cache
and lead to lower performance.
The defaults may be unsuitable for a file system that re
quires a very large number of i-nodes or is intended to hold a large num
ber of very small files. Such a file system should be created with an
8K or 4K block size. This also requires you to specify a smaller fragment
size. We recommend always using a fragment size that is 1/8 the block
size (less testing has been done on other fragment size factors). The
newfs(8) options for this would be ``newfs -f 1024 -b 8192 ...''.
If a large partition is intended to be used to hold fewer,
larger files, such as database files, you can increase the bytes/i-node
ratio which reduces the number of i-nodes (maximum number of files and
directories that can be created) for that partition. Decreasing the
number of inodes in a file system can greatly reduce fsck(8) recovery
times after a crash. Do not use this option unless you are actually stor
ing large files on the partition, because if you overcompensate you
can wind up with a file system that has lots of free space remaining but
cannot accommodate any more files. Using 32768, 65536, or 262144
bytes/i-node is recommended. You can go higher but it will have only in
cremental effects on fsck(8) recovery times. For example, ``newfs -i
32768 ...''.
tunefs(8) may be used to further tune a file system. This
command can be run in single-user mode without having to reformat the file
system. However, this is possibly the most abused program in the sys
tem. Many people attempt to increase available file system space by set
ting the minfree percentage to 0. This can lead to severe file system
fragmentation and we do not recommend that you do this. Really the only
tunefs(8) option worthwhile here is turning on softupdates with
``tunefs -n enable /filesystem''. (Note: in FreeBSD 4.5 and later, softupdates
can be turned on using the -U option to newfs(8), and sysinstall(8)
will typically enable softupdates automatically for non-root file
systems). Softupdates drastically improves meta-data performance, mainly
file creation and deletion. We recommend enabling softupdates on most
file systems; however, there are two limitations to softupdates that you
should be aware of when determining whether to use it on a file sys
tem. First, softupdates guarantees file system consistency in the case
of a crash but could very easily be several seconds (even a minute!) behind
on pending write to the physical disk. If you crash you may lose more
work than otherwise. Secondly, softupdates delays the freeing of file
system blocks. If you have a file system (such as the root file
system) which is close to full, doing a major update of it, e.g. ``make
installworld'', can run it out of space and cause the update to fail. For
this reason, softupdates will not be enabled on the root file system dur
ing a typical install. There is no loss of performance since the root
file system is rarely written to.
A number of run-time mount(8) options exist that can help
you tune the
system. The most obvious and most dangerous one is async.
Do not ever use it; it is far too dangerous. A less dangerous and more
useful
mount(8) option is called noatime. UNIX file systems nor
mally update the last-accessed time of a file or directory whenever it is ac
cessed. This operation is handled in FreeBSD with a delayed write and
normally does not create a burden on the system. However, if your system
is accessing a huge number of files on a continuing basis the buffer
cache can wind up getting polluted with atime updates, creating a burden on
the system. For example, if you are running a heavily loaded web site,
or a news server with lots of readers, you might want to consider
turning off atime updates on your larger partitions with this mount(8) option.
However, you should not gratuitously turn off atime updates every
where. For example, the /var file system customarily holds mailboxes, and
atime (in combination with mtime) is used to determine whether a mailbox
has new mail. You might as well leave atime turned on for mostly read-only
partitions such as / and /usr as well. This is especially useful for /
since some system utilities use the atime field for reporting.

STRIPING DISKS

In larger systems you can stripe partitions from several
drives together to create a much larger overall partition. Striping can al
so improve the performance of a file system by splitting I/O operations
across two or more disks. The vinum(8) and ccdconfig(8) utilities may be
used to create simple striped file systems. Generally speaking, strip
ing smaller partitions such as the root and /var/tmp, or essentially
read-only partitions such as /usr is a complete waste of time. You should
only stripe partitions that require serious I/O performance, typically
/var, /home, or custom partitions used to hold databases and web pages.
Choosing the proper stripe size is also important. File systems tend to
store metadata on power-of-2 boundaries and you usually want to reduce
seeking rather than increase seeking. This means you want to use a
large offcenter stripe size such as 1152 sectors so sequential I/O
does not seek both disks and so meta-data is distributed across both disks
rather than concentrated on a single disk. If you really need to get
sophisticated, we recommend using a real hardware RAID controller from the
list of FreeBSD supported controllers.

SYSCTL TUNING

sysctl(8) variables permit system behavior to be monitored
and controlled at run-time. Some sysctls simply report on the behavior of
the system; others allow the system behavior to be modified; some may be
set at boot time using rc.conf(5), but most will be set via
sysctl.conf(5). There are several hundred sysctls in the system, including many
that appear to be candidates for tuning but actually are not. In this doc
ument we will only cover the ones that have the greatest effect on the
system.
The kern.ipc.shm_use_phys sysctl defaults to 0 (off) and may
be set to 0 (off) or 1 (on). Setting this parameter to 1 will cause all
System V shared memory segments to be mapped to unpageable physical
RAM. This feature only has an effect if you are either (A) mapping
small amounts of shared memory across many (hundreds) of processes, or (B)
mapping large amounts of shared memory across any number of processes.
This feature allows the kernel to remove a great deal of internal memory
management page-tracking overhead at the cost of wiring the shared mem
ory into core, making it unswappable.
The vfs.vmiodirenable sysctl defaults to 1 (on). This pa
rameter 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 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 amount of memory. Turn
ing on this sysctl allows the buffer cache to use the VM Page Cache to
cache the directories. The advantage is that all of memory is now
available for caching directories. The disadvantage is that the minimum
in-core memory used to cache a directory is the physical page size (typi
cally 4K) rather than 512 bytes. We recommend turning this option off in
memory-constrained environments; however, when on, it will substan
tially improve the performance of services that manipulate a large number
of files. Such services can include web caches, large mail systems,
and news systems. Turning on this option will generally not reduce per
formance even with the wasted memory but you should experiment to find
out.
The vfs.write_behind sysctl defaults to 1 (on). This tells
the file system 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.
The vfs.hirunningspace sysctl determines how much outstand
ing 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 occurring at the same time.
There are various other buffer-cache and VM page cache re
lated sysctls. We do not recommend modifying these values. As of FreeBSD
4.3, the VM system does an extremely good job tuning itself.
The net.inet.tcp.sendspace and net.inet.tcp.recvspace
sysctls are of particular interest if you are running network intensive appli
cations. They control the amount of send and receive buffer space allowed
for any given TCP connection. The default sending buffer is 32K; the de
fault receiving buffer is 64K. You can often improve bandwidth utilization
by increasing the default at the cost of eating up more kernel memory for
each connection. We do not recommend increasing the defaults if you
are serving hundreds or thousands of simultaneous connections because it
is possible to quickly run the system out of memory due to stalled con
nections building up. But if you need high bandwidth over a fewer number
of connections, especially if you have gigabit Ethernet, increasing
these defaults can make a huge difference. You can adjust the buffer size
for incoming and outgoing data separately. For example, if your machine
is primarily doing web serving you may want to decrease the recvspace in
order to be able to increase the sendspace without eating too much ker
nel memory. Note that the routing table (see route(8)) can be used to
introduce route-specific send and receive buffer size defaults.
As an additional management tool you can use pipes in your
firewall rules (see ipfw(8)) to limit the bandwidth going to or from par
ticular IP blocks or ports. For example, if you have a T1 you might
want to limit your web traffic to 70% of the T1's bandwidth in order to
leave the remainder available for mail and interactive use. Normally
a heavily loaded web server will not introduce significant latencies
into other services even if the network link is maxed out, but enforc
ing a limit can smooth things out and lead to longer term stability. Many
people also enforce artificial bandwidth limitations in order to ensure
that they are not charged for using too much bandwidth.
Setting the send or receive TCP buffer to values larger than
65535 will result in a marginal performance improvement unless both
hosts support the window scaling extension of the TCP protocol, which is
controlled by the net.inet.tcp.rfc1323 sysctl. These extensions should be
enabled and the TCP buffer size should be set to a value larger than
65536 in order to obtain good performance from certain types of network
links; specifically, gigabit WAN links and high-latency satellite links.
RFC1323 support is enabled by default.
The net.inet.tcp.always_keepalive sysctl determines whether
or not the TCP implementation should attempt to detect dead TCP connec
tions by intermittently delivering ``keepalives'' on the connection.
By default, this is enabled for all applications; by setting this sysctl
to 0, only applications that specifically request keepalives will use
them. In most environments, TCP keepalives will improve the management of
system state by expiring dead TCP connections, particularly for systems
serving dialup users who may not always terminate individual TCP connec
tions before disconnecting from the network. However, in some environments,
temporary network outages may be incorrectly identified as dead ses
sions, resulting in unexpectedly terminated TCP connections. In such envi
ronments, setting the sysctl to 0 may reduce the occurrence of TCP ses
sion disconnections.
The net.inet.tcp.delayed_ack TCP feature is largely misun
derstood. Historically speaking, this feature was designed to allow the
acknowledgement to transmitted data to be returned along with the re
sponse. For example, when you type over a remote shell, the acknowledge
ment to the character you send can be returned along with the data rep
resenting the echo of the character. With delayed acks turned off, the
acknowledgement may be sent in its own packet, before the remote service has
a chance to echo the data it just received. This same concept also ap
plies to any interactive protocol (e.g. SMTP, WWW, POP3), and can cut the
number of tiny packets flowing across the network in half. The FreeB
SD delayed ACK implementation also follows the TCP protocol rule that at
least every other packet be acknowledged even if the standard 100ms
timeout has not yet passed. Normally the worst a delayed ACK can do is
slightly delay the teardown of a connection, or slightly delay the ramp-up
of a slowstart TCP connection. While we are not sure we believe that
the several FAQs related to packages such as SAMBA and SQUID which ad
vise turning off delayed acks may be referring to the slow-start issue. In
FreeBSD, it would be more beneficial to increase the slow-start flight
size via the
net.inet.tcp.slowstart_flightsize sysctl rather than disable
delayed acks.
The net.inet.tcp.inflight.enable sysctl turns on bandwidth
delay product limiting for all TCP connections. The system will attempt
to calculate the bandwidth delay product for each connection and limit
the amount of data queued to the network to just the amount required to
maintain optimum throughput. This feature is useful if you are serving
data over modems, GigE, or high speed WAN links (or any other link
with a high bandwidth*delay product), especially if you are also using
window scaling or have configured a large send window. If you enable this
option, you should also be sure to set net.inet.tcp.inflight.debug to 0
(disable debugging), and for production use setting
net.inet.tcp.inflight.min to at least 6144 may be beneficial. Note however, that setting
high minimums may effectively disable bandwidth limiting depending on
the link. The limiting feature reduces the amount of data built up in
intermediate router and switch packet queues as well as reduces the
amount of data built up in the local host's interface queue. With fewer
packets queued up, interactive connections, especially over slow modems,
will also be able to operate with lower round trip times. However, note
that this feature only affects data transmission (uploading / server
side). It does not affect data reception (downloading).
Adjusting net.inet.tcp.inflight.stab is not recommended.
This parameter defaults to 20, representing 2 maximal packets added to the
bandwidth delay product window calculation. The additional window is
required to stabilize the algorithm and improve responsiveness to chang
ing conditions, but it can also result in higher ping times over slow
links (though still much lower than you would get without the in
flight algorithm). In such cases you may wish to try reducing this pa
rameter to 15, 10, or 5, and you may also have to reduce
net.inet.tcp.inflight.min (for example, to 3500) to get the desired effect. Reducing these
parameters should be done as a last resort only.
The net.inet.ip.portrange.* sysctls control the port number
ranges automatically bound to TCP and UDP sockets. There are three
ranges: a low range, a default range, and a high range, selectable via the
IP_PORTRANGE setsockopt(2) call. Most network programs use the default
range which is controlled by net.inet.ip.portrange.first and
net.inet.ip.portrange.last, which default to 49152 and 65535, respectively. Bound port
ranges are used for outgoing connections, and it is possible to run the
system out of ports under certain circumstances. This most commonly
occurs when you are running a heavily loaded web proxy. The port range is
not an issue when running a server which handles mainly incoming connec
tions, such as a normal web server, or has a limited number of outgoing
connections, such as a mail relay. For situations where you may run out
of ports, we recommend decreasing net.inet.ip.portrange.first modestly.
A range of 10000 to 30000 ports may be reasonable. You should also
consider firewall effects when changing the port range. Some firewalls
may block large ranges of ports (usually low-numbered ports) and ex
pect systems to use higher ranges of ports for outgoing connections. By de
fault net.inet.ip.portrange.last is set at the maximum allowable
port number.
The kern.ipc.somaxconn sysctl limits the size of the listen
queue for accepting new TCP connections. The default value of 128 is
typically too low for robust handling of new connections in a heavily
loaded web server environment. For such environments, we recommend increasing
this value to 1024 or higher. The service daemon may itself limit the
listen queue size (e.g. sendmail(8), apache) but will often have a direc
tive in its configuration file to adjust the queue size up. Larger lis
ten queues also do a better job of fending off denial of service at
tacks.
The kern.maxfiles sysctl determines how many open files the
system supports. The default is typically a few thousand but you may
need to bump this up to ten or twenty thousand if you are running
databases or large descriptor-heavy daemons. The read-only kern.openfiles
sysctl may be interrogated to determine the current number of open files
on the system.
The vm.swap_idle_enabled sysctl is useful in large multi-us
er systems where you have lots of users entering and leaving the system
and lots of idle processes. Such systems tend to generate a great deal
of continuous pressure on free memory reserves. Turning this feature on
and adjusting the swapout hysteresis (in idle seconds) via
vm.swap_idle_threshold1 and
vm.swap_idle_threshold2 allows you to depress the priority
of pages associated with idle processes more quickly then the normal pa
geout algorithm. This gives a helping hand to the pageout daemon. Do
not turn this option on unless you need it, because the tradeoff you
are making is to essentially pre-page memory sooner rather than later,
eating more swap and disk bandwidth. In a small system this option will have
a detrimental effect but in a large system that is already doing mod
erate paging this option allows the VM system to stage whole processes
into and out of memory more easily.

LOADER TUNABLES

Some aspects of the system behavior may not be tunable at
runtime because memory allocations they perform must occur early in the boot
process. To change loader tunables, you must set their values in load
er.conf(5) and reboot the system.
kern.maxusers controls the scaling of a number of static
system tables, including defaults for the maximum number of open files,
sizing of network memory resources, etc. As of FreeBSD 4.5,
kern.maxusers is automatically sized at boot based on the amount of memory available
in the system, and may be determined at run-time by inspecting the
value of the read-only kern.maxusers sysctl. Some sites will require
larger or smaller values of kern.maxusers and may set it as a loader
tunable; values of 64, 128, and 256 are not uncommon. We do not recom
mend going above 256 unless you need a huge number of file descriptors;
many of the tunable values set to their defaults by kern.maxusers may be
individually overridden at boot-time or run-time as described elsewhere
in this document. Systems older than FreeBSD 4.4 must set this value
via the kernel
config(8) option maxusers instead.
kern.ipc.nmbclusters may be adjusted to increase the number
of network mbufs the system is willing to allocate. Each cluster rep
resents approximately 2K of memory, so a value of 1024 represents 2M of
kernel memory reserved for network buffers. You can do a simple calcula
tion to figure out how many you need. If you have a web server which maxes
out at 1000 simultaneous connections, and each connection eats a 16K re
ceive and 16K send buffer, you need approximately 32MB worth of network
buffers to deal with it. A good rule of thumb is to multiply by 2, so
32MBx2 = 64MB/2K = 32768. So for this case you would want to set
kern.ipc.nmbclusters to 32768. We recommend values between 1024 and 4096 for ma
chines with moderates amount of memory, and between 4096 and 32768 for ma
chines with greater amounts of memory. Under no circumstances should
you specify an arbitrarily high value for this parameter, it could lead to
a boot-time crash. The -m option to netstat(1) may be used to observe
network cluster use. Older versions of FreeBSD do not have this tunable
and require that the kernel config(8) option NMBCLUSTERS be set instead.
More and more programs are using the sendfile(2) system call
to transmit files over the network. The kern.ipc.nsfbufs sysctl con
trols the number of file system buffers sendfile(2) is allowed to use to per
form its work. This parameter nominally scales with kern.maxusers so you
should not need to modify this parameter except under extreme circumstances.
See the TUNING section in the sendfile(2) manual page for details.

KERNEL CONFIG TUNING

There are a number of kernel options that you may have to
fiddle with in a large-scale system. In order to change these options you
need to be able to compile a new kernel from source. The config(8)
manual page and the handbook are good starting points for learning how to do
this. Generally the first thing you do when creating your own custom
kernel is to strip out all the drivers and services you do not use. Re
moving things like INET6 and drivers you do not have will reduce the size
of your kernel, sometimes by a megabyte or more, leaving more memory
available for applications.
SCSI_DELAY may be used to reduce system boot times. The de
faults are fairly high and can be responsible for 5+ seconds of delay
in the boot process. Reducing SCSI_DELAY to something below 5 seconds
could work (especially with modern drives).
There are a number of *_CPU options that can be commented
out. If you only want the kernel to run on a Pentium class CPU, you can
easily remove I486_CPU, but only remove I586_CPU if you are sure your CPU
is being recognized as a Pentium II or better. Some clones may be rec
ognized as a Pentium or even a 486 and not be able to boot without those
options. If it works, great! The operating system will be able to bet
ter use higherend CPU features for MMU, task switching, timebase, and even
device operations. Additionally, higher-end CPUs support 4MB MMU
pages, which the kernel uses to map the kernel itself into memory, increasing
its efficiency under heavy syscall loads.

IDE WRITE CACHING

FreeBSD 4.3 flirted with turning off IDE write caching.
This reduced write bandwidth to IDE disks but was considered necessary
due to serious data consistency issues introduced by hard drive vendors.
Basically the problem is that IDE drives lie about when a write completes.
With IDE write caching turned on, IDE hard drives will not only write
data to disk out of order, they will sometimes delay some of the blocks
indefinitely under heavy disk load. A crash or power failure can result
in serious file system corruption. So our default was changed to be
safe. Unfortunately, the result was such a huge loss in performance that
we caved in and changed the default back to on after the release. You
should check the default on your system by observing the hw.ata.wc sysctl
variable. If IDE write caching is turned off, you can turn it back on
by setting the hw.ata.wc loader tunable to 1. More information on tun
ing the ATA driver system may be found in the ata(4) manual page. If
you need performance, go with SCSI.

CPU, MEMORY, DISK, NETWORK

The type of tuning you do depends heavily on where your sys
tem begins to bottleneck as load increases. If your system runs out of
CPU (idle times are perpetually 0%) then you need to consider upgrading the
CPU or moving to an SMP motherboard (multiple CPU's), or perhaps you need
to revisit the programs that are causing the load and try to optimize
them. If your system is paging to swap a lot you need to consider adding
more memory. If your system is saturating the disk you typically see high
CPU idle times and total disk saturation. systat(1) can be used to
monitor this. There are many solutions to saturated disks: increasing mem
ory for caching, mirroring disks, distributing operations across
several machines, and so forth. If disk performance is an issue and
you are using IDE drives, switching to SCSI can help a great deal.
While modern IDE drives compare with SCSI in raw sequential bandwidth,
the moment you start seeking around the disk SCSI drives usually win.
Finally, you might run out of network suds. The first line
of defense for improving network performance is to make sure you are
using switches instead of hubs, especially these days where switches are
almost as cheap. Hubs have severe problems under heavy loads due to
collision back-off and one bad host can severely degrade the entire
LAN. Second, optimize the network path as much as possible. For example,
in firewall(7) we describe a firewall protecting internal hosts
with a topology where the externally visible hosts are not routed
through it. Use 100BaseT rather than 10BaseT, or use 1000BaseT rather
than 100BaseT, depending on your needs. Most bottlenecks occur at the WAN
link (e.g. modem, T1, DSL, whatever). If expanding the link is not an
option it may be possible to use the dummynet(4) feature to implement peak
shaving or other forms of traffic shaping to prevent the overloaded
service (such as web services) from affecting other services (such as email),
or vice versa. In home installations this could be used to give in
teractive traffic (your browser, ssh(1) logins) priority over services
you export from your box (web services, email).

HISTORY

The tuning manual page was originally written by Matthew
Dillon and first appeared in FreeBSD 4.3, May 2001.
BSD June 25, 2002

docs.sk

comprehensive documentation repository

See also

tuning(7)

NAME

SYSTEM SETUP - DISKLABEL, NEWFS, TUNEFS, SWAP

STRIPING DISKS

SYSCTL TUNING

LOADER TUNABLES

KERNEL CONFIG TUNING

IDE WRITE CACHING

CPU, MEMORY, DISK, NETWORK

SEE ALSO

HISTORY