Re: "Enhanced" MD code avaible for review
From: Jeff Garzik
Date: Wed Mar 17 2004 - 14:21:12 EST
Justin T. Gibbs wrote:
[ I tried sending this last night from my Adaptec email address and have
yet to see it on the list. Sorry if this is dup for any of you. ]
Included linux-kernel in the CC (and also bounced this post there).
For the past few months, Adaptec Inc, has been working to enhance MD.
The FAQ from several corners is going to be "why not DM?", so I would
humbly request that you (or Scott Long) re-post some of that rationale
here...
The goals of this project are:
o Allow fully pluggable meta-data modules
yep, needed
o Add support for Adaptec ASR (aka HostRAID) and DDF
(Disk Data Format) meta-data types. Both of these
formats are understood natively by certain vendor
BIOSes meaning that arrays can be booted from transparently.
yep, needed
For those who don't know, DDF is particularly interesting. A storage
industry association, "SNIA", has gotten most of the software and
hardware RAID folks to agree on a common, vendor-neutral on-disk format.
Pretty historic, IMO :) Since this will be appearing on most of the
future RAID hardware, Linux users will be left out in a big way if this
isn't supported.
EARLY DRAFT spec for DDF was posted on snia.org at
http://www.snia.org/tech_activities/ddftwg/DDFTrial-UseDraft_0_45.pdf
o Improve the ability of MD to auto-configure arrays.
hmmmm. Maybe in my language this means "improve ability for low-level
drivers to communicate RAID support to upper layers"?
o Support multi-level arrays transparently yet allow
proper event notification across levels when the
topology is known to MD.
I'll need to see the code to understand what this means, much less
whether it is needed ;-)
o Create a more generic "work item" framework which is
used to support array initialization, rebuild, and
verify operations as well as miscellaneous tasks that
a meta-data or RAID personality may need to perform
from a thread context (e.g. spare activation where
meta-data records may need to be sequenced carefully).
This is interesting. (guessing) sort of like a pluggable finite state
machine?
o Modify the MD ioctl interface to allow the creation
of management utilities that are meta-data format
agnostic.
I'm thinking that for 2.6, it is much better to use a more tightly
defined interface via a Linux character driver. Userland write(2)'s
packets of data (h/w raid commands or software raid configuration
commands), and read(2)'s the responses.
ioctl's are a pain for 32->64-bit translation layers. Using a
read/write interface allows one to create an interface that requires no
translation layer -- a big deal for AMD64 and IA32e processors moving
forward -- and it also gives one a lot more control over the interface.
See, we need what I described _anyway_, as a chrdev-based interface to
sending and receiving ATA taskfiles or SCSI cdb's.
It would be IMO simple to extend this to a looks-a-lot-like-ioctl
raid_op interface.
A snapshot of this work is now available here:
http://people.freebsd.org/~gibbs/linux/SRC/emd-0.7.0-tar.gz
Your email didn't say... this appears to be for 2.6, correct?
This snapshot includes support for RAID0, RAID1, and the Adaptec
ASR and DDF meta-data formats. Additional RAID personalities and
support for the Super90 and Super 1 meta-data formats will be added
in the coming weeks, the end goal being to provide a superset of
the functionality in the current MD.
groovy
Since the current MD notification scheme does not allow MD to receive
notifications unless it is statically compiled into the kernel, we
would like to work with the community to develop a more generic
notification scheme to which modules, such as MD, can dynamically
register. Until that occurs, these EMD snapshots will require at
least md.c to be a static component of the kernel.
You would just need a small stub that holds a notifier pointer, yes?
Architectural Notes
===================
The major areas of change in "EMD" can be categorized into:
1) "Object Oriented" Data structure changes
These changes are the basis for allowing RAID personalities
to transparently operate on "disks" or "arrays" as member
objects. While it has always been possible to create
multi-level arrays in MD using block layer stacking, our
approach allows MD to also stack internally. Once a given
RAID or meta-data personality is converted to the new
structures, this "feature" comes at no cost. The benefit
to stacking internally, which requires a meta-data format
that supports this, is that array state can propagate up
and down the topology without the loss of information
inherent in using the block layer to traverse levels of an
array.
I have a feeling that consensus will prefer that we fix the block layer,
and then figure out the best way to support "automatic stacking" --
since DDF and presumeably other RAID formats will require automatic
setup of raid0+1, etc.
Are there RAID-specific issues here, that do not apply to e.g.
multipathing, which I've heard needs more information at the block layer?
2) Opcode based interfaces.
Rather than add additional method vectors to either the
RAID personality or meta-data personality objects, the new
code uses only a few methods that are parameterized. This
has allowed us to create a fairly rich interface between
the core and the personalities without overly bloating
personality "classes".
Modulo what I said above, about the chrdev userland interface, we want
to avoid this. You're already going down the wrong road by creating
more untyped interfaces...
static int raid0_raidop(mdk_member_t *member, int op, void *arg)
{
switch (op) {
case MDK_RAID_OP_MSTATE_CHANGED:
The preferred model is to create a single marshalling module (a la
net/core/ethtool.c) that converts the ioctls we must support into a
fully typed function call interface (a la struct ethtool_ops).
3) WorkItems
Workitems provide a generic framework for queuing work to
a thread context. Workitems include a "control" method as
well as a "handler" method. This separation allows, for
example, a RAID personality to use the generic sync handler
while trapping the "open", "close", and "free" of any sync
workitems. Since both handlers can be tailored to the
individual workitem that is queued, this removes the need
to overload one or more interfaces in the personalities.
It also means that any code in MD can make use of this
framework - it is not tied to particular objects or modules
in the system.
Makes sense, though I wonder if we'll want to make this more generic.
hardware RAID drivers might want to use this sort of stuff internally?
4) "Syncable Volume" Support
All of the transaction accounting necessary to support
redundant arrays has been abstracted out into a few inline
functions. With the inclusion of a "sync support" structure
in a RAID personality's private data structure area and the
use of these functions, the generic sync framework is fully
available. The sync algorithm is also now more like that
in 2.4.X - with some updates to improve performance. Two
contiguous sync ranges are employed so that sync I/O can
be pending while the lock range is extended and new sync
I/O is stalled waiting for normal I/O writes that might
conflict with the new range complete. The syncer updates
its stats more frequently than in the past so that it can
more quickly react to changes in the normal I/O load. Syncer
backoff is also disabled anytime there is pending I/O blocked
on the syncer's locked region. RAID personalities have
full control over the size of the sync windows used so that
they can be optimized based on RAID layout policy.
interesting. makes sense on the surface, I'll have to think some more...
5) IOCTL Interface
"EMD" now performs all of its configuration via an "mdctl"
character device. Since one of our goals is to remove any
knowledge of meta-data type in the user control programs,
initial meta-data stamping and configuration validation
occurs in the kernel. In general, the meta-data modules
already need this validation code in order to support
auto-configuration, so adding this capability adds little
to the overall size of EMD. It does, however, require a
few additional ioctls to support things like querying the
maximum "coerced" size of a disk targeted for a new array,
or enumerating the names of installed meta-data modules,
etc.
This area of EMD is still in very active development and we expect
to provide a drop of an "emdadm" utility later this week.
I haven't evaluated yet the ioctl interface. I do understand the need
to play alongside the existing md interface, but if there are huge
numbers of additions, it would be preferred to just use the chrdev
straightaway. Such a chrdev would be easily portable to 2.4.x kernels
too :)
7) Correction of RAID0 Transform
The RAID0 transform's "merge function" assumes that the
incoming bio's starting sector is the same as what will be
presented to its make_request function. In the case of a
partitioned MD device, the starting sector is shifted by
the partition offset for the target offset. Unfortunately,
the merge functions are not notified of the partition
transform, so RAID0 would often reject requests that span
"chunk" boundaries once shifted. The fix employed here is
to determine if a partition transform will occur and take
this into account in the merge function.
interesting
Adaptec is currently validating EMD through formal testing while
continuing the build-out of new features. Our hope is to gather
feedback from the Linux community and adjust our approach to satisfy
the community's requirements. We look forward to your comments,
suggestions, and review of this project.
Thanks much for working with the Linux community.
One overall comment on merging into 2.6: the patch will need to be
broken up into pieces. It's OK if each piece is dependent on the prior
one, and it's OK if there are 20, 30, even 100 pieces. It helps a lot
for review to see the evolution, and it also helps flush out problems
you might not have even noticed. e.g.
- add concept of member, and related helper functions
- use member functions/structs in raid drivers raid0.c, etc.
- fix raid0 transform
- add ioctls needed in order for DDF to be useful
- add DDF format
etc.
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