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.\"
.\" swapcache - Cache clean filesystem data & meta-data on SSD-based swap
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.Dd February 7, 2010
.Dt SWAPCACHE 8
.Os
.Sh NAME
.Nm swapcache
.Nd a
mechanism which allows the system to use fast swap to cache filesystem
data and meta-data.
.Sh SYNOPSIS (defaults shown)
.Cd sysctl vm.swapcache.accrate=100000
.Cd sysctl vm.swapcache.maxfilesize=0
.Cd sysctl vm.swapcache.maxburst=2000000000
.Cd sysctl vm.swapcache.curburst=4000000000
.Cd sysctl vm.swapcache.minburst=10000000
.Cd sysctl vm.swapcache.read_enable=0
.Cd sysctl vm.swapcache.meta_enable=0
.Cd sysctl vm.swapcache.data_enable=0
.Cd sysctl vm.swapcache.maxlaunder=256
.Sh DESCRIPTION
.Nm
is a system capability which allows a solid state disk (SSD) in a swap
space configuration to be used to cache clean filesystem data and meta-data
in addition to its normal function backing anonymous memory.
.Pp
Sysctls are used to manage operational parameters and can be adjusted at
any time.  Typically a large initial burst is desired after system boot,
controlled by the initial
.Cd vm.swapcache.curburst
parameter.
This parameter is reduced as data is written to swap by the swapcache
and increased at a rate specified by
.Cd vm.swapcache.accrate .
Once this parameter reaches zero write activity ceases until it has
recovered sufficiently for write activity to resume.
.Pp
.Cd vm.swapcache.meta_enable
enables the writing of filesystem meta-data to the swapcache.  Filesystem
metadata is any data which the filesystem accesses via the disk device
using buffercache.
.Pp
.Cd vm.swapcache.data_enable
enables the writing of filesystem file-data to the swapcache.  Filesystem
filedata is any data which the filesystem accesses via a regular file.
In technical terms, when the buffer cache is used to access a regular
file through its vnode.  Please do not blindly turn on this option,
see the PERFORMANCE TUNING section for more information.
.Pp
.Cd vm.swapcache.read_enable
enables reading from the swapcache and should be set to 1 for normal
operation.
.Pp
.Cd vm.swapcache.maxfilesize
controls which files are to be cached based on their size.
If set to non-zero only files smaller than the specified size
will be cached.  Larger files will not be cached.
.Sh PERFORMANCE TUNING
Best operation is achieved when the active data set fits within the
swapcache.
.Pp
.Bl -tag -width 4n -compact
.It Cd vm.swapcache.accrate
This specifies the burst accumulation rate in bytes per second and
ultimately controls the write bandwidth to swap averaged over a long
period of time.
This parameter must be carefully chosen to manage the write endurance of
the SSD in order to avoid wearing it out too quickly.
Even though SSDs have limited write endurance there is massive
cost/performance benefit to using one in a swapcache configuration.
.Pp
Lets use the Intel X25V 40G MLC SATA SSD as an example.  This device
has approximately a 40TB (40 terrabyte) write endurance.
Limiting the long term average bandwidth to 100K/sec leads to no more
than ~9G/day writing which calculates approximately to a 12 year
endurance.
Endurance scales linearly with size.  The 80G version of this SSD
will have a write endurance of approximately 80TB.
.Pp
MLC SSDs have approximately a 1000x write endurance, while the
lower density higher-cost SLC SSDs have an approximately 100000x
write endurance.  MLC SSDs can be used for the swapcache (and swap)
as long as the system manager is cognizant of its limitations.
.Pp
.It Cd vm.swapcache.meta_enable
Turning on just
.Cd meta_enable
causes only filesystem meta-data to be cached and will result
in very fast directory operations even over millions of inodes.
.Pp
.It Cd vm.swapcache.data_enable
Turning on
.Cd data_enable
(with or without other features) allows bulk file data to be
cached.
This feature is very useful for web server operation when the
operational data set fits in swap.
The usefulness is somewhat mitigated by the maximum number
of vnodes supported by the system via
.Cd kern.maxfiles ,
because the bulk data in the cache is lost when the related
vnode is recycled.  In this case it might be desireable to
take the plunge into running a 64-bit kernel which can support
far more vnodes.  32-bit kernels have limited kernel virtual
memory (KVM) and cannot reliably support more than around
100,000 active vnodes.  64-bit kernels can support 300,000+
active vnodes.
.Pp
Data caching is definitely more wasteful of SSD write bandwidth
than meta-data caching.  It doesn't hurt performance per-say,
but may cause the
.Nm
to exhaust its burst and smack against the long term average
bandwidth limit, causing the SSD to wear out at the maximum rate you
programmed.  Data caching is far less wasteful and more efficient
if (on a 64-bit system only) you provide a sufficiently large SSD and
increase
.Cd kern.maxvnodes
to cover the entire directory topology being served.
Each vnode requires about 1K of physical ram.
.Pp
.It Cd vm.swapcache.maxfilesize
This may be used to reduce cache thrashing when a focus on a small
potentially fragmented filespace is desired, leaving the
larger files alone.
.Pp
.It Cd vm.swapcache.minburst
This controls hysteresis and prevents nickle-and-dime write bursting.
Once
.Cd curburst
drops to zero writing to the swapcache ceases until it has recovered
past
.Cd minburst .
The idea here is to avoid creating a heavily fragmented swapcache where
reading data from a file must alternate between the cache and the primary
filesystem.  Doing so does not save disk seeks on the primary filesystem
so we want to avoid doing small bursts.  This parameter allows us to do
larger bursts.
The larger bursts also tend to improve SSD performance as the SSD itself
can do a better job write-combining and erasing blocks.
.Pp
.El
.Pp
Finally, interleaved swap (multiple SSDs) may be used to increase
performance even further.  A single SATA SSD is typically capable of
reading 120-220MB/sec.  Configuring two SSDs for your swap will
improve aggregate swapcache read performance by 1.5x to 1.8x.
In tests with two Intel 40G SSDs 300MB/sec was easily achieved.
.Pp
At this point you will be configuring more swap space than a 32 bit
.Dx
kernel can handle (due to KVM limitations).  By default, 32 bit
.Dx
systems only support 32G of configured swap and while this limit
can be increased somewhat in
.Pa /boot/loader.conf
you should really be using a 64-bit
.Dx
kernel instead.  64-bit systems support up to 512G of swap by default
and can be boosted to up to 8TB if you are really crazy and have enough ram.
Each 1GB of swap requires around 1MB of physical memory to manage it so
the practical 'reasonable' limit is more around 1TB of swap.
.Pp
Of course, a 1TB SSD is something on the order of $3000+ as of this writing.
Even though these quantities might not be cost effective, storage levels
more in the 100-200G range certainly are.  If the machine has only a 1GigE
ethernet (100MB/s) there's no point configuring it for more SSD bandwidth.
A single SSD of the desired size would be sufficient.
.Sh INITIAL BURSTING & REPEATED BURSTING
Even though the average write bandwidth is limited it is desireable
to have a large initial burst after boot to load the cache.
.Cd curburst
is initialized to 4GB by default and you can force rebursting
by adjusting it with a sysctl.
Remember that
.Cd curburst
dynamically tracks burst and will go up and down depending.
.Pp
In addition there will be periods of time where the system is in
steady state and not writing to the swapcache.  During these periods
.Cd curburst
will inch back up but will not exceed
.Cd maxburst .
Thus the
.Cd maxburst
value controls how large a repeated burst can be.
.Pp
A second bursting parameter called
.Cd vm.swapcache.minburst
controls bursting when the maximum write bandwidth has been reached.
When
.Cd minburst
reaches zero write activity ceases and
.Cd curburst
is allowed to recover up to
.Cd minburst
before write activity resumes.  The recommended range for the
.Cd minburst
parameter is 1MB to 50MB.  This parameter has a relationship to
how fragmented the swapcache gets when not in a steady state.
Large bursts reduce fragmentation and reduce incidences of
excessive seeking on the hard drive.  If set too low the
swapcache will become fragmented within a single regular file
and the constant back-and-forth between the swapcache and the
hard drive will result in excessive seeking on the hard drive.
.Sh SWAPCACHE SIZE & MANAGEMENT
The swapcache feature will use up to 75% of configured swap space.
The remaining 25% is reserved for normal paging operation.
The system operator should configure at least 4 x SWAP verses
main memory and no less than 8G of swap space.
If a 40G SSD is used the recommendation is to configure 16G to 32G of
swap (note: 32-bit is limited to 32G of swap by default, for 64-bit
it is 512G of swap).
.Pp
If swapcache reaches the 75% limit it will begin tearing down swap
in linear bursts by iterating through available VM objects, until
swap space use drops to 70%.  The tear-down is limited by the rate at
which new data is written and this rate in turn is often limited
by
.Cd vm.swapcache.accrate ,
resulting in an orderly replacement of cached data and meta-data.
The limit is typically only reached when doing full data+meta-data
caching with no file size limitations and serving primarily large
files, or (on a 64-bit system) bumping kern.maxvnodes up to very
high values.
.Sh NORMAL SWAP PAGING ACTIVITY WITH SSD SWAP
This is not a function of
.Nm
per-say but instead a normal function of the system.  Most systems have
sufficient memory that they do not need to page memory to swap.  These
types of systems are the ones best suited for MLC SSD configured swap
running with a
.Nm
configuration.
Systems which modestly page to swap, in the range of a few hundred
megabytes a day worth of writing, are also well suited for MLC SSD
configured swap.  Desktops usually fall into this category even if they
page out a bit more because swap activity is governed by the actions of
a single person.
.Pp
Systems which page anonymous memory heavily when
.Nm
would otherwise be turned off are not usually well suited for MLC SSD
configured swap.  Heavy paging activity is not governed by
.Nm
bandwidth control parameters and can lead to excessive uncontrolled
writing to the MLC SSD, causing premature wearout.  You would have to
use the lower density, more expensive SLC SSD technology (which has 100x
the durability per GB).  This isn't to say that
.Nm
would be ineffective, just that the aggregate write bandwidth required
to support the system would be too large for MLC flash technologies.
.Pp
With this caveats in mind SSD based paging on systems with insufficient
ram can be extremely effective in extending the useful life of the system.
For example, a system with a measily 192MB of ram can run a -j 8 parallel
build world in a little less than twice the time it would take if the system
had 2G of ram when SSD swap is configured, whereas it would take 5x to 10x
as long with normal HD based swap.
.Sh WARNING
SSDs have limited durability and
.Nm
parameters should be carefully chosen to avoid early wearout.
For example, the Intel X25V 40G SSD has a nominal 40TB (terrabyte)
write durability.
Generally speaking you want to select parameters that will give you
at least 5 years of service life.  10 years is a good compromise.
.Pp
Durability typically scales with size and also depends on the
wear-leveling algorithm used by the device.  Durability can often
be improved by configuring less space (in a manufacturer-fresh drive)
than the drive's capacity.  For example, by only using 32G of a 40G
SSD.
.Pp
The swapcache is designed for use with SSDs configured as swap and
will generally not improve performance when a normal hard drive is used
for swap.
.Pp
.Nm smartctl
(from pkgsrc's smartmontools) may be used to retrieve the wear indicator
from the drive.
One usually runs something like 'smartctl -d sat -a /dev/daXX'
(for AHCI/SILI/SCSI), or 'smartctl -a /dev/adXX' for NATA.  Many SSDs
will brick the SATA port when smart operations are done while the drive
is busy with normal activity, so the tool should only be run when the
SSD is idle.
.Pp
The wear-out meter is entry 233 (0xe9) in the list.
It usually starts at 99 and decrements over time until it reaches 0, at
which point writes to the SSD drive will begin failing.
Wear on SSDs is a function only of the write rate... the write durability.
Power-on hours, power cycles, and read operations do not effect wear.
.Pp
SSD's with MLC-based flash technology are high-density, low-cost solutions
with limited write durability.  SLC-based flash technology is a low-density,
higher-cost solution with 100x the write durability as MLC.  The durability
also scales with the amount of flash storage, with SLC based flash typically
twice as expensive per gigabyte.  From a cost perspective SLC based flash
is 50x more cost effective in situations where high write bandwidths are
required.  MLC is at least 2x more cost effective in situations where
high write bandwidths are not required.
.Nm
is usable with both technologies.
.Sh SEE ALSO
.Xr swapon 8 ,
.Xr fstab 5
.Sh HISTORY
.Nm
first appeared in
.Dx 2.5 .
.Sh AUTHORS
.An Matthew Dillon