Editing Containers/Mini-summit 2008 notes

[[Category: Containers]]

Intros (8:36am)

        Dave Hansen
        Eric Biederman
        Jason Byron, Red Hat
        Joe Ruscio, Evergrid
        Joe McDonald
        HP China
        Sonny Rao
        HP
        HP
        Matine Silberman HP
        Sandy Harris
        NEC Japan
        John Schultz, AOL
        Pavel Emelyanov, Parallels/OpenVZ
        Denis Lunev, Parallels/OpenVZ
        Andrey Mirkin, Parallels/OpenVZ
        Constant Chan
        Benjamin Thery, Bull
        Daniel Lezcano, IBM
        Serge Hallyn, IBM
        Oren Laadan, Columbia University

On Phone:
        Amy Griffis, HP
        Dhaval Giani, IBM
        Peter Zijlstra

(Later walk-ins):
        Paul Menage, Google

== Namespaces and containers ==

Why do various companies want containers?
        ibm: workload management
        EB: using containers as improved chroot
        HP: wants similar to ibm, plus security
        parallels: hosted providers

sysfs issues
        EB gives status: should go into next merge window

mini-namespaces
        NFS
                clients should behave differently on diff. containers
                currently uses single sunrpc transport for all containers
        Dave: is there a list of all openvz mini-ns?
        EB:
                proposal:
                        create little filesystems
                        still store everything in nsproxy
                currently:
                        some people want same process in different netns's
                        almost possible now, but can't open new sockets
                namespace enter:
                        3 purposes
                                login
                                monitoring
                                configuring
                may be worth prototyping the proposal
                        address mqns, or sunrpc, or fuse?
        DH:
                openvz addresses this using one big clone(), right?
                (yes)

userid namespaces
        EB summarizes his proposal
                userid ns is unsharable without privilege
                userids, capabilities, security labels become ns-local
                hierarchical like pidns
        openvz: just does chroot
        DH:
                observers that system vs. app containers have different requirements
        EB:
                so with userid namespaces, user has god-like powers over created namespaces
        EB+SH will talk about hacking something this week during ols
        Uses:
                user unttrusted mounts
                build systems

device namespaces
        tty namespaces rejected
        should be solved with generic device namespaces
                virtualize the major:minor->device mapping
        reserved device numbers (unnamed)
                created with /proc?
                get_unnamed_device()
        tty ideas:
                use selinux ptys
                use user namespaces
                use legacy ptys
                leverage ptyfs
        Suka is not on, so he gets volunteered to do pure /dev/pts fs approach

per-container LSMs:
        SH: thinks LSMs should handle it
        EB:
                original purpose of chroot
                set up policies from inside container
                creating smack container inside selinux would be ideal

entering a  container
        netns: identified using pid of a ns
        sh: can we solve this using EB's namespace filesystems proposal?
        (EB goes to the board to demonstrate his proposal)
        PM: Can we use control groups?
        PE: Can we re-use /proc/pid/ ?
        EB: could have a ns with no processes in it
        Example of command using this:
                ip set eth0 netns <pid>
                becomes
                ip set eth0 netns /proc/<pid>/
        DL:
                a real netns problem is knowing when a childns has died
                the netnsfs mount could solve that
        PE: EB, can you send POC patches for the namespace?
                EB and EM will both send their own POC.

DL: people have complained about needing CAP_SYS_ADMIN to unshare ns
        EB: example, setuid root sysvipc-using program could be fooled

PE: Entering a container:
        reasons:
                monitoring
                enter an administrative command
        DH: how do you do it now?
        PE: numerical ID for each VE, use it to enter
        EB:
                one need for entering: /sbin/hotplug
        (someone): does hijack suffice?
        EB: two cases:
                partial entering
                full entering
                sys_hijack does not address partial entering
        DH:
                why need partial entering?
                fs stuff can be done without entering
        PM: privileged process
        PE:
                will look at hijack patches
                someone will re-send hijack to containers@
                EB:
                        if we can do sys_hijack cleanly,
                        we can use it to solve kthread problem

== Checkpoint/Restart [CR] ==

=== Uses of CR ===

* '''migration and live migration:'''  e.g. for load balancing, maintenance, clusters and SSIs, etc. may or may not assume a shared file system between endpoints

* '''suspend/resume (aka hibernation):''' e.g. for hibernation, gang-scheduling and priority running, OS maintenance

* '''failure recovery / fault tolerance:''' periodic checkpoints, and restart from most recent (unlike the previous scenarios, here the applications continue to execute after the checkpoint, perhaps modify the file system)

* '''time-travel:''' periodic checkpoints and restart from any previous checkpoint (here, too, attention is required to capturing the state of the file system as well)

* [PE] '''fast-launch:''' reduce start-up time of heavy applications by restarting from a preset checkpoint instead of launching from scratch.

* [EB] '''remote fork:''' e.g. in a cluster

(the last two scenarios are likely to require adjustments during,
or after, the restart to tolerate changes in the file system or
otherwise in the environment)

* [EB,OL] '''distributed checkpoint:''' the ability to checkpoint and restart a distributed application
across multiple nodes as a whole.

EB reminded that at the last kernel summit nobody complained about the
wish to add CR capabilities to the kernel. The issue was and remains
related to technical choices. 

=== General design ===

* '''Kernel-space vs user-space'''

OL: the issue of kernel-space vs. user-space is pivotal to design.
kernel support is mandatory to provide completeness and transparency.
Even the recent experience with "cryo" demonstrated that users-space
requires the kernel to expose a very fine-grained API.

Everyone agreed to aim at a monolithic interface, such that nearly
all of the CR will be done in the kernel. The kernel will return
(checkpoint) or receive (restart) a blob with the image of the state
of the container.

* '''Kernel-module ?'''

OL: can we implement mostly in a kernel module and then move CR into
the kernel later ?

EB: better to add CR functionality gradually directly to the kernel.

* '''Compatibility between kernels'''

DLu: there is an issue with compatibility between kernels - even same
kernel compiled with different options and/or compiler, and also if
the kernel ABI changes.

OL: suggest to use an intermediate representation for the checkpoint
image to avoid the issue as much as possible; conversion, if needed,
will take place with userland tools. No aim to bridge ABI changes in
case of migration: instead, fail the restart. 

EB: format the blob such that userland tools it will be possible to 
parse it and easily detect a version/configuration mismatch. 

* '''Streaming checkpoint image ?'''

DLu: using sequential file (non seek-able) like a socket for the
checkpoint image is a challenge.

OL: with proper planning it is not complicated to achieve, and it has
advantage of possible to pass through a filter, e.g. for compression,
encryption, format conversion etc.

* '''Checkpoint operation'''

The procedure will entail five steps:
# Pre-dump
# Freeze the container
# Dump
# Thaw/Kill the container
# Post-dump

"pre-dump" works before freezing the container, e.g. the pre-copy for
live migration and minimize application downtime.

"post-dump" works after the container resumes execution, e.g. in the
case of a checkpoint (not migration) write-back the data to secondary
storage, again to minimize application downtime.

OL: we should be able to checkpoint from inside the container, keep 
that in mind for later (also relates to the freezer).

* '''Restart operation'''

Restart is done by first creating a container, then creating the
process tree in it, and then each process restores its own state. 
This allows to re-use existing kernel code (e.g., restoring a memory
region is a simple matter of calling mmap() and populating it). 

OL: suggest that the process tree be created in userspace. 

DLu: prefer to do everything, including process creation, in the 
kernel, his experience shows that it isn't difficult.

* '''Error recovery'''

Should checkpoint fail, the container should continue execution
without noticing it. If either checkpoint or restart fail, there 
should be a way to inform the caller/user of the reason (something
more informative than -EBUSY). 

=== Road plan ===

A this point we want to create a proof of concept and CR a simple
application. We will add iteratively more and more kernel resources.

The first items to address:
# Create a container object (the context on which CR operates)
# Extend the container freezer cgroup  ?)
# Interface via syscall or ioctl ?

First step - a simple application:
a single process, not using any files, no signal pending, no IPC etc.
Need to save state (registers, IDs), memory maps and contents (except
for read-only portions, e.g. text).
Assume that the file system state doesn't change between checkpoint
and restart.

Next steps:
# process hierarchy and relationships (multiple tasks and zombies)
# multiple threads (and shared memory)
# open files: regular file, fifo, pipe, socket-pair
# signals, timers
# TBD

=== Documentation ===

DH: proof of concept requires explicit documentation of what can be
checkpointed and what cannot be checkpointed, as well as what will
be the error returned in response to a failure.