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Subject[RFC] DDRaid higher level cluster raid
Date
Greetings,

I am pleased to be able to present today an interesting project that has kept
me busy for the last couple of months.

DDRaid is a cluster block device that, together with a cluster filesystem like
GFS, gives you the ability to operate a "distributed data cluster" where the
cluster data is distributed redundantly over the nodes of a cluster rather
than using a single, shared disk. You could also use ddraid with iscsi or
fiber channel disks, and it even works reasonably well as a local software
raid. But the interesting thing about it to me is the distributed data
aspect.

As far as I know, ddraid is the first higher level cluster raid, or if that is
not correct, it is certainly the first to appear as open source. It is based
on Raid 3.5, a simple raid scheme I investigated earlier, and presented a
paper on at Linux Kongress 2002:

http://sourceware.org/cluster/ddraid/raid35.pdf

Raid 3.5 has the attractive property that it can be implemented without any
caching or read-before-write, which is very important for a cluster. Cluster
caching is a wretchedly complex affair that is normally implemented at a
higher level by the cluster filesystem and/or vfs. We certainly do not want
to have two wretchedly complex layers of cluster caching if we can avoid it.
This is what you would get by extending Raid 5, say, to operate across a
cluster.

My Raid 3.5 scheme turned out to work pretty well. Some initial benchmarks
were posted yesterday, here:

https://www.redhat.com/archives/linux-cluster/2005-March/msg00112.html

The executive summary is that on an ideal linear load, ddraid runs about 62%
faster than a single raw disk. An example of such a linear load is copying a
large file. On random IO loads, ddraid performs no worse than a single raw
disk. Of course, increased performance is only the secondary goal of ddraid.
The primary goal is data redundancy.

Further details on ddraid were provided in the initial project announcement,
and I will not repeat them here:

https://www.redhat.com/archives/linux-cluster/2005-March/msg00034.html

My purpose today is twofold: to solicit feedback on some of the kernel issues
in the ddraid driver, and to introduce some relatively approachable cluster
code that is easy to install and try out, even if you don't have a cluster.
In other words, I would like to begin the process of involving more of the
kernel community in cluster issues. The ddraid driver is a rather nice test
case for this, because it touches on most of the interesting cluster issues
without being particularly big and complex.

Let me start by defining the difference between a cluster block device and a
non-cluster block device. It is not necessarily what you would think. For
example, you can export a block device over the network, but that does not
make it a cluster block device: you can still only mount one filesystem at a
time on it.

Here are some of the things we expect of a cluster block device:

* Since multiple nodes can access the device simultaneously, the cluster
block device may need to prevent these accesses from interfering in
situations that the cluster filesystem itself has no knowledge of and
therefore cannot handle.

* If the cluster block device has its own metadata, access to the metadata
must be synchronized across the cluster

* Cluster control: The cluster block device needs to respond to management
commands arriving from other nodes. For example, so that a instance of
the device may be created simultaneously on all nodes of the cluster, and
each instance will know how to access the same underlying hardware
resources.

* Fault tolerance: If the block device relies on services provided by other
nodes, those services need to be able to fail over to other nodes in the
event a node fails. If a connection is temporarily broken, the cluster
block device should be able to resume operation without failing any IO.

A cluster block device does not need to or should not provide:

* Caching and cache synchronization. Except for its own metadata, a cluster
block device should let the cluster filesystem and vfs take care of this.

* Multiple access. Every block device already provides this, albeit not
necessarily safely.

A cluster block device may use a cluster lock manager (e.g., gdlm) to
implement whatever synchronization it needs. I did not use this approach
myself. Instead I used streaming message based synchronization over standard
sockets, something like DBus. I did this for efficiency, but it also has the
attractive side effect of avoiding a dependency on any particular cluster
lock manager. Instead I depend only on sockets.

Which brings up an issue. I implement socket failover by arranging for a
userspace process to open a new link and pass it to the kernel driver using
SCM_RIGHTS. I don't think I can do that with netlink. So I use PF_UNIX, and
kludge that to work in-kernel with what you might call user-space-in-kernel
hacks. I would like to clean this up one way or another. I would appreciate
feedback both on the strategy of passing a socket link to the kernel, and how
I might clean up the PF_UNIX interface if that turns out to be the only way
to do it.

Here is the ddraid kernel patch. Look for SCM_RIGHTS, appreciate the full
unadulterated ugliness:

http://sourceware.org/cluster/ddraid/ddraid-2.6.11.3

Besides passing socket fds to the kernel, I use the PF_UNIX interface to
control the raid device and to allow it to report error conditions such as
broken links. I use an anonymous socket for this purpose, which in itself is
completely insecure. I presume that I must pass credentials in order to
secure this link, which is not yet in the patch. Or is there some other
standard way?

I like the convenience of anonymous sockets quite a lot. However, it is not
clear to me how to prevent or deal with collisions in the anonymous socket
name space. I would appreciate guidance on that. I could always fall back
to filesystem-based sockets, though I do not like having to delete the socket
before using it. The current code will work with either.

One big problem I find with sockets is shutting them down reliably, so that
daemons waiting on them will unblock and the device mapper device can be
removed. The userspace socket shutdown facility relies on signals.
In-kernel, I would have to explicitly field signals in order to make this
interface work. This falls rather short of elegant. I would like to do
something about the socket shutdown problem. It would be very nice to be
able to shut down a socket simply and reliably from within the kernel.

A huge, horrible, gaping wound of a problem, far from limited to ddraid, is
memory inversion. DDRaid uses a userspace server for synchronization. The
server may try to allocate working memory under low memory conditions, but
the server sits in the block IO path. If memory happens to be full of dirty
data that needs to be written out over the ddraid device, we are in trouble.
Moving the server into the kernel would not avoid the problem, because the
real problem is that a process in PF_MEMALLOC state is being serviced by a
process not in PF_MEMALLOC state, which just happens to be a user space
process. We could easily create a similar situation entirely within the
kernel, and in fact, the ddraid driver is full of such situations.

Something coherent needs to be done about this. This is not an easy problem
at all, and ddraid is far from the only kernel code that suffers from it.

Here is a tarball, complete with kernel patch:

http://sourceware.org/cluster/ddraid/ddraid.0.0.1.tgz

This is a snapshot as of yesterday's benchmarks. There is little or no
documentation on how to build and operate this subsystem, a deficiency I will
correct shortly. The tarball is designed to be unpacked into the root of a
2.6.11.3 tree. Userspace code will end up in a subdirectory of drivers/md,
which is not where it is supposed to live permanently, but is how I prefer to
work at this point. Test code is driven by the same makefile that builds the
userspace code. The makefile is about all there is for documentation.

The code is in various states of disrepair, especially the ddraid server which
was a quick hack done over a few days, and not simple at all. How it works
is an intersting subject in its own right. The kernel code itself has
moments of lucidity, but is also terminally crufty in places. It reinvents
some infrastructure that already exists, like work queues. There are bugs,
two that I know of. SMP hasn't been tested at all, nor has big endianness
(which is guaranteed not to work) or 64 bit builds. There are masses of
unhandled error conditions. All that said, it is functional code, if you
hold your tongue right.

The ddraid project page is here:

http://sourceware.org/cluster/ddraid

If you are going to be at LCA in Canberra next month, I cordially invite you
to attend my talk, where I will present a paper on ddraid. (Rusty, if you
are reading, it _was_ supposed to be a cluster mirror paper, but it evolved.)

Regards,

Daniel
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