RAMSTER HOW-TO
Author: Dan Magenheimer
Ramster maintainer: Konrad Wilk <konrad.wilk@oracle.com>
This is a HOWTO document for ramster which, as of this writing, is in
the kernel as a subdirectory of zcache in drivers/staging, called ramster.
(Zcache can be built with or without ramster functionality.) If enabled
and properly configured, ramster allows memory capacity load balancing
across multiple machines in a cluster. Further, the ramster code serves
as an example of asynchronous access for zcache (as well as cleancache and
frontswap) that may prove useful for future transcendent memory
implementations, such as KVM and NVRAM. While ramster works today on
any network connection that supports kernel sockets, its features may
become more interesting on future high-speed fabrics/interconnects.
Ramster requires both kernel and userland support. The userland support,
called ramster-tools, is known to work with EL6-based distros, but is a
set of poorly-hacked slightly-modified cluster tools based on ocfs2, which
includes an init file, a config file, and a userland binary that interfaces
to the kernel. This state of userland support reflects the abysmal userland
skills of this suitably-embarrassed author; any help/patches to turn
ramster-tools into more distributable rpms/debs useful for a wider range
of distros would be appreciated. The source RPM that can be used as a
starting point is available at:
http://oss.oracle.com/projects/tmem/files/RAMster/
As a result of this author's ignorance, userland setup described in this
HOWTO assumes an EL6 distro and is described in EL6 syntax. Apologies
if this offends anyone!
Kernel support has only been tested on x86_64. Systems with an active
ocfs2 filesystem should work, but since ramster leverages a lot of
code from ocfs2, there may be latent issues. A kernel configuration that
includes CONFIG_OCFS2_FS should build OK, and should certainly run OK
if no ocfs2 filesystem is mounted.
This HOWTO demonstrates memory capacity load balancing for a two-node
cluster, where one node called the "local" node becomes overcommitted
and the other node called the "remote" node provides additional RAM
capacity for use by the local node. Ramster is capable of more complex
topologies; see the last section titled "ADVANCED RAMSTER TOPOLOGIES".
If you find any terms in this HOWTO unfamiliar or don't understand the
motivation for ramster, the following LWN reading is recommended:
-- Transcendent Memory in a Nutshell (lwn.net/Articles/454795)
-- The future calculus of memory management (lwn.net/Articles/475681)
And since ramster is built on top of zcache, this article may be helpful:
-- In-kernel memory compression (lwn.net/Articles/545244)
Now that you've memorized the contents of those articles, let's get started!
A. PRELIMINARY
1) Install two x86_64 Linux systems that are known to work when
upgraded to a recent upstream Linux kernel version.
On each system:
2) Configure, build and install, then boot Linux, just to ensure it
can be done with an unmodified upstream kernel. Confirm you booted
the upstream kernel with "uname -a".
3) If you plan to do any performance testing or unless you plan to
test only swapping, the "WasActive" patch is also highly recommended.
(Search lkml.org for WasActive, apply the patch, rebuild your kernel.)
For a demo or simple testing, the patch can be ignored.
4) Install ramster-tools as root. An x86_64 rpm for EL6-based systems
can be found at:
http://oss.oracle.com/projects/tmem/files/RAMster/
(Sorry but for now, non-EL6 users must recreate ramster-tools on
their own from source. See above.)
5) Ensure that debugfs is mounted at each boot. Examples below assume it
is mounted at /sys/kernel/debug.
B. BUILDING RAMSTER INTO THE KERNEL
Do the following on each system:
1) Using the kernel configuration mechanism of your choice, change
your config to include:
CONFIG_CLEANCACHE=y
CONFIG_FRONTSWAP=y
CONFIG_STAGING=y
CONFIG_CONFIGFS_FS=y # NOTE: MUST BE y, not m
CONFIG_ZCACHE=y
CONFIG_RAMSTER=y
For a linux-3.10 or later kernel, you should also set:
CONFIG_ZCACHE_DEBUG=y
CONFIG_RAMSTER_DEBUG=y
Before building the kernel please doublecheck your kernel config
file to ensure all of the settings are correct.
2) Build this kernel and change your boot file (e.g. /etc/grub.conf)
so that the new kernel will boot.
3) Add "zcache" and "ramster" as kernel boot parameters for the new kernel.
4) Reboot each system approximately simultaneously.
5) Check dmesg to ensure there are some messages from ramster, prefixed
by "ramster:"
# dmesg | grep ramster
You should also see a lot of files in:
# ls /sys/kernel/debug/zcache
# ls /sys/kernel/debug/ramster
These are mostly counters for various zcache and ramster activities.
You should also see files in:
# ls /sys/kernel/mm/ramster
These are sysfs files that control ramster as we shall see.
Ramster now will act as a single-system zcache on each system
but doesn't yet know anything about the cluster so can't yet do
anything remotely.
C. CONFIGURING THE RAMSTER CLUSTER
This part can be error prone unless you are familiar with clustering
filesystems. We need to describe the cluster in a /etc/ramster.conf
file and the init scripts that parse it are extremely picky about
the syntax.
1) Create a /etc/ramster.conf file and ensure it is identical on both
systems. This file mimics the ocfs2 format and there is a good amount
of documentation that can be searched for ocfs2.conf, but you can use:
cluster:
name = ramster
node_count = 2
node:
name = system1
cluster = ramster
number = 0
ip_address = my.ip.ad.r1
ip_port = 7777
node:
name = system2
cluster = ramster
number = 1
ip_address = my.ip.ad.r2
ip_port = 7777
You must ensure that the "name" field in the file exactly matches
the output of "hostname" on each system; if "hostname" shows a
fully-qualified hostname, ensure the name is fully qualified in
/etc/ramster.conf. Obviously, substitute my.ip.ad.rx with proper
ip addresses.
2) Enable the ramster service and configure it. If you used the
EL6 ramster-tools, this would be:
# chkconfig --add ramster
# service ramster configure
Set "load on boot" to "y", cluster to start is "ramster" (or whatever
name you chose in ramster.conf), heartbeat dead threshold as "500",
network idle timeout as "1000000". Leave the others as default.
3) Reboot both systems. After reboot, try (assuming EL6 ramster-tools):
# service ramster status
You should see "Checking RAMSTER cluster "ramster": Online". If you do
not, something is wrong and ramster will not work. Note that you
should also see that the driver for "configfs" is loaded and mounted,
the driver for ocfs2_dlmfs is not loaded, and some numbers for network
parameters. You will also see "Checking RAMSTER heartbeat: Not active".
That's all OK.
4) Now you need to start the cluster heartbeat; the cluster is not "up"
until all nodes detect a heartbeat. In a real cluster, heartbeat detection
is done via a cluster filesystem, but ramster doesn't require one. Some
hack-y kernel code in ramster can start the heartbeat for you though if
you tell it what nodes are "up". To enable the heartbeat, do:
# echo 0 > /sys/kernel/mm/ramster/manual_node_up
# echo 1 > /sys/kernel/mm/ramster/manual_node_up
This must be done on BOTH nodes and, to avoid timeouts, must be done
approximately concurrently on both nodes. On an EL6 system, it is
convenient to put these lines in /etc/rc.local. To confirm that the
cluster is now up, on both systems do:
# dmesg | grep ramster
You should see ramster "Accepted connection" messages in dmesg on both
nodes after this. Note that if you check userland status again with
# service ramster status
you will still see "Checking RAMSTER heartbeat: Not active". That's
still OK... the ramster kernel heartbeat hack doesn't communicate to
userland.
5) You now must tell each node the node to which it should "remotify" pages.
On this two node cluster, we will assume the "local" node, node 0, has
memory overcommitted and will use ramster to utilize RAM capacity on
the "remote node", node 1. To configure this, on node 0, you do:
# echo 1 > /sys/kernel/mm/ramster/remote_target_nodenum
You should see "ramster: node 1 set as remotification target" in dmesg
on node 0. Again, on EL6, /etc/rc.local is a good place to put this
on node 0 so you don't forget to do it at each boot.
6) One more step: By default, the ramster code does not "remotify" any
pages; this is primarily for testing purposes, but sometimes it is
useful. This may change in the future, but for now, on node 0, you do:
# echo 1 > /sys/kernel/mm/ramster/pers_remotify_enable
# echo 1 > /sys/kernel/mm/ramster/eph_remotify_enable
The first enables remotifying swap (persistent, aka frontswap) pages,
the second enables remotifying of page cache (ephemeral, cleancache)
pages.
On EL6, these lines can also be put in /etc/rc.local (AFTER the
node_up lines), or at the beginning of a script that runs a workload.
7) Note that most testing has been done with both/all machines booted
roughly simultaneously to avoid cluster timeouts. Ideally, you should
do this too unless you are trying to break ramster rather than just
use it. ;-)
D. TESTING RAMSTER
1) Note that ramster has no value unless pages get "remotified". For
swap/frontswap/persistent pages, this doesn't happen unless/until
the workload would cause swapping to occur, at which point pages
are put into frontswap/zcache, and the remotification thread starts
working. To get to the point where the system swaps, you either
need a workload for which the working set exceeds the RAM in the
system; or you need to somehow reduce the amount of RAM one of
the system sees. This latter is easy when testing in a VM, but
harder on physical systems. In some cases, "mem=xxxM" on the
kernel command line restricts memory, but for some values of xxx
the kernel may fail to boot. One may also try creating a fixed
RAMdisk, doing nothing with it, but ensuring that it eats up a fixed
amount of RAM.
2) To see if ramster is working, on the "remote node", node 1, try:
# grep . /sys/kernel/debug/ramster/foreign_*
# # note, that is space-dot-space between grep and the pathname
to monitor the number (and max) ephemeral and persistent pages
that ramster has sent. If these stay at zero, ramster is not working
either because the workload on the local node (node 0) isn't creating
enough memory pressure or because "remotifying" isn't working. On the
local system, node 0, you can watch lots of useful information also.
Try:
grep . /sys/kernel/debug/zcache/*pageframes* \
/sys/kernel/debug/zcache/*zbytes* \
/sys/kernel/debug/zcache/*zpages* \
/sys/kernel/debug/ramster/*remote*
Of particular note are the remote_*_pages_succ_get counters. These
show how many disk reads and/or disk writes have been avoided on the
overcommitted local system by storing pages remotely using ramster.
At the risk of information overload, you can also grep:
/sys/kernel/debug/cleancache/* and /sys/kernel/debug/frontswap/*
These show, for example, how many disk reads and/or disk writes have
been avoided by using zcache to optimize RAM on the local system.
AUTOMATIC SWAP REPATRIATION
You may notice that while the systems are idle, the foreign persistent
page count on the remote machine slowly decreases. This is because
ramster implements "frontswap selfshrinking": When possible, swap
pages that have been remotified are slowly repatriated to the local
machine. This is so that local RAM can be used when possible and
so that, in case of remote machine crash, the probability of loss
of data is reduced.
REBOOTING / POWEROFF
If a system is shut down while some of its swap pages still reside
on a remote system, the system may lock up during the shutdown
sequence. This will occur if the network is shut down before the
swap mechansim is shut down, which is the default ordering on many
distros. To avoid this annoying problem, simply shut off the swap
subsystem before starting the shutdown sequence, e.g.:
# swapoff -a
# reboot
Ideally, this swapoff-before-ifdown ordering should be enforced permanently
using shutdown scripts.
KNOWN PROBLEMS
1) You may periodically see messages such as:
ramster_r2net, message length problem
This is harmless but indicates that a node is sending messages
containing compressed pages that exceed the maximum for zcache
(PAGE_SIZE*15/16). The sender side needs to be fixed.
2) If you see a "No longer connected to node..." message or a "No connection
established with node X after N seconds", it is possible you may
be in an unrecoverable state. If you are certain all of the
appropriate cluster configuration steps described above have been
performed, try rebooting the two servers concurrently to see if
the cluster starts.
Note that "Connection to node... shutdown, state 7" is an intermediate
connection state. As long as you later see "Accepted connection", the
intermediate states are harmless.
3) There are known issues in counting certain values. As a result
you may see periodic warnings from the kernel. Almost always you
will see "ramster: bad accounting for XXX". There are also "WARN_ONCE"
messages. If you see kernel warnings with a tombstone, please report
them. They are harmless but reflect bugs that need to be eventually fixed.
ADVANCED RAMSTER TOPOLOGIES
The kernel code for ramster can support up to eight nodes in a cluster,
but no testing has been done with more than three nodes.
In the example described above, the "remote" node serves as a RAM
overflow for the "local" node. This can be made symmetric by appropriate
settings of the sysfs remote_target_nodenum file. For example, by setting:
# echo 1 > /sys/kernel/mm/ramster/remote_target_nodenum
on node 0, and
# echo 0 > /sys/kernel/mm/ramster/remote_target_nodenum
on node 1, each node can serve as a RAM overflow for the other.
For more than two nodes, a "RAM server" can be configured. For a
three node system, set:
# echo 0 > /sys/kernel/mm/ramster/remote_target_nodenum
on node 1, and
# echo 0 > /sys/kernel/mm/ramster/remote_target_nodenum
on node 2. Then node 0 is a RAM server for node 1 and node 2.
In this implementation of ramster, any remote node is potentially a single
point of failure (SPOF). Though the probability of failure is reduced
by automatic swap repatriation (see above), a proposed future enhancement
to ramster improves high-availability for the cluster by sending a copy
of each page of date to two other nodes. Patches welcome!