kernel - Update swapcache manual page

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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
40TB (40 terabyte) write endurance, but see later
notes on this, it is more a minimum value.
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 a 1000-10000x write endurance, while the lower density
higher-cost SLC SSDs have an approximately 10000-100000x 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
and even in the face of other invasive operations being run
by other processes.
.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 se,
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
When data caching is turned on you generally want to use
.Xr chflags 1
with the
.Cm cache
flag to enable data caching on a directory.
This flag is tracked by the namecache and does not need to be
recursively set in the directory tree.
Simply setting the flag in a top level directory is sufficient.
A typical setup is something like this:
.Pp
.Dl chflags cache /etc /sbin /bin /usr /home
.Dl chflags noscache /usr/obj
.Pp
Alternatively if you have NFS mounts where chflags does not work you
can enable caching in some parent directory, then selectively disable
it.
.Pp
.Dl chflags cache /
.Dl chflags noscache /usr/obj /tmp /var/tmp
.Pp
If that doesn't work you can turn off
.Cd vm.swapcache.use_chflags
entirely and not bother with any chflagging.
.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 nickel-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
.It Cd vm_swapcache.maxswappct
This controls the maximum amount of swapspace
.Nm
may use, in percentage terms.
.El
.Pp
It is important to note that you should always use
.Xr disklabel64 8
to label your SSD.  Disklabel64 will properly align the base of the
partition space relative to the physical drive regardless of how badly
aligned the fdisk slice is.
This will significantly reduce write amplification and write combining
inefficiencies on the SSD.
.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 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 a 1TB configuration 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
by default.
The remaining 25% is reserved for normal paging operation.
The system operator should configure at least 4 times the SWAP space
versus 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), and to leave the remainder unwritten and unused.
.Pp
The
.Cd vm_swapcache.maxswappct
sysctl may be used to change the default.
You may have to change this default if you also use
.Xr tmpfs 5 ,
.Xr vn 4 ,
or if you have not allocated enough swap for reasonable normal paging
activity to occur (in which case you probably shouldn't be using
.Nm
anyway).
.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 se 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 10x
the durability).  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 caveat 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 measly 192MB of ram and SSD swap 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, whereas it would take 5x to 10x
as long with normal HD based swap.
.Sh WARNINGS
I am going to repeat and expand a bit on SSD wear.
Wear on SSDs is a function of the write durability of the cells,
whether the SSD implements static or dynamic wear leveling, and
write amplification effects based on the type of write activity.
Write amplification occurs due to wasted space when the SSD must
erase and rewrite the underlying flash blocks.  e.g. MLC flash uses
128KB erase/write blocks.
.Pp
.Nm
parameters should be carefully chosen to avoid early wearout.
For example, the Intel X25V 40G SSD has a minimum write durability
of 40TB and an actual durability that can be quite a bit higher.
Generally speaking, you want to select parameters that will give you
at least 10 years of service life.
The most important parameter to control this is
.Cd vm.swapcache.accrate .
.Nm
uses a very conservative 100KB/sec default but even a small X25V
can probably handle 300KB/sec of continuous writing and still last
10 years.
.Pp
Depending on the wear leveling algorithm the drive uses, durability
and performance can sometimes be improved by configuring less
space (in a manufacturer-fresh drive) than the drive's probed capacity.
For example, by only using 32G of a 40G SSD.
SSDs typically implement 10% more storage than advertised and
use this storage to improve wear leveling.  As cells begin to fail
this overallotment slowly becomes part of the primary storage
until it has been exhausted.  After that the SSD has basically failed.
Keep in mind that if you use a larger portion of the SSD's advertised
storage the SSD will not know if/when you decide to use less unless
appropriate TRIM commands are sent (if supported), or a low level
factory erase is issued.
.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 sysutils/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.  Some SSDs
(particularly the Intels) 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
ID 232 (0xe8) in the SMART data dump indicates available reserved
space and ID 233 (0xe9) is the wear-out meter.  Reserved space
typically starts at 100 and decrements to 10, after which the SSD
is considered to operate in a degraded mode.  The wear-out meter
typically starts at 99 and decrements to 0, after which the SSD
has failed.
.Pp
.Nm
tends to use large 64K writes and tends to cluster multiple writes
linearly.  The SSD is able to take significant advantage of this
and write amplification effects are greatly reduced.  If we
take a 40G Intel X25V as an example the vendor specifies a write
durability of approximately 40TB, but
.Nm
should be able to squeeze out upwards of 200TB due the fairly optimal
write clustering it does.
The theoretical limit for the Intel X25V is 400TB (10,000 erase cycles
per MLC cell, 40G drive), but the firmware doesn't do perfect static
wear leveling so the actual durability is less.
.Pp
In contrast, most filesystems directly stored on a SSD have
fairly severe write amplification effects and will have durabilities
ranging closer to the vendor-specified limit.
Power-on hours, power cycles, and read operations do not really affect
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 10x the write durability as MLC.  The durability
also scales with the amount of flash storage.  SLC based flash is typically
twice as expensive per gigabyte.  From a cost perspective, SLC based flash
is at least 5x more cost effective in situations where high write
bandwidths are required (because it lasts 10x longer).  MLC is at least
2x more cost effective in situations where high write bandwidth is not
required.
When wear calculations are in years, these differences become huge, but
often the quantity of storage needed trumps the wear life so we expect most
people will be using MLC.
.Nm
is usable with both technologies.
.Sh SEE ALSO
.Xr swapon 8 ,
.Xr disklabel64 8 ,
.Xr fstab 5
.Sh HISTORY
.Nm
first appeared in
.Dx 2.5 .
.Sh AUTHORS
.An Matthew Dillon