Date:	Mon, 20 May 2013 07:55:35 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	devel@...uxdriverproject.org, linux-kernel@...r.kernel.org,
	gregkh@...uxfoundation.org, linux-mm@...ck.org,
	konrad.wilk@...cle.com, liwanp@...ux.vnet.ibm.com,
	bob.liu@...cle.com
Subject: RE: [PATCH] staging: ramster: add how-to document

Hey Greg --

Since this is documentation only and documents existing
behavior, I'm not clear whether it is acceptable for
an rcN release in the current cycle or must wait until
the next window.  Since it is a new file, it should
apply to either so I'll leave the choice up to you.

Thanks,
Dan

> From: Dan Magenheimer [mailto:dan.magenheimer@...cle.com]
> Sent: Monday, May 20, 2013 8:52 AM
> To: devel@...uxdriverproject.org; linux-kernel@...r.kernel.org; gregkh@...uxfoundation.org; linux-
> mm@...ck.org; konrad.wilk@...cle.com; dan.magenheimer@...cle.com; liwanp@...ux.vnet.ibm.com;
> bob.liu@...cle.com
> Subject: [PATCH] staging: ramster: add how-to document
> 
> Add how-to documentation that provides a step-by-step guide
> for configuring and trying out a ramster cluster.
> 
> Signed-off-by: Dan Magenheimer <dan.magenheimer@...cle.com>
> ---
>  drivers/staging/zcache/ramster/ramster-howto.txt |  366 ++++++++++++++++++++++
>  1 files changed, 366 insertions(+), 0 deletions(-)
>  create mode 100644 drivers/staging/zcache/ramster/ramster-howto.txt
> 
> diff --git a/drivers/staging/zcache/ramster/ramster-howto.txt
> b/drivers/staging/zcache/ramster/ramster-howto.txt
> new file mode 100644
> index 0000000..7b1ee3b
> --- /dev/null
> +++ b/drivers/staging/zcache/ramster/ramster-howto.txt
> @@ -0,0 +1,366 @@
> +			RAMSTER HOW-TO
> +
> +Author: Dan Magenheimer
> +Ramster maintainer: Konrad Wilk <konrad.wilk@...cle.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!
> --
> 1.7.1
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