Contrasting DRBD with md/nbd

From: Neil Brown
Date: Thu May 14 2009 - 02:31:39 EST

[ cc: list massively trimmed compared to original posting of code and
subsequent discussion..]


Prior to giving DRBD a proper review I've been trying to make sure
that I understand it, so I have a valid model to compare the code
against (and so I can steal any bits that I like for md:-)

The model I have been pondering is an extension of the md/raid1 + nbd
model. Understanding exactly what would need to be added to that
model to provide identical services will help (me, at least)
understand DRBD.

So I thought I would share the model with you all in case it helps
anyone else, and in case there are any significant error that need to
be corrected.

Again, this is *not* how DRBD is implemented - it describes an
alternate implementation that would provide the same functionality.

In this model there is something like md/raid1, and something like
nbd. The raid1 communicates with both (all) drives via the same nbd
interface (which in a real implementation would be optimised to
bypass the socket layer for a local device). This is different to
current md/raid1+nbd installations which only use nbd to access the
remote device.

The enhanced NBD

The 'nbd' server accepts connections from 2 (or more) clients and
co-ordinates IO. Apart from the "obvious" of servicing read and write requests, sending
acknowledgements and handling barriers, the particular
responsibilities of the nbd server are:
- to detect and resolve concurrent writes
- to maintain a bitmap recording "the blocks which have been
written to this device but not to (all) the other device(s).

Concurrent writes

To detect concurrent writes it needs a little bit of help from the
raid1 module. Whenever raid1 is about to issue a write, it
sends a reservation request to one of the nbd devices (typically the
local one) to record that the write is in-flight. Then it sends the
write to all devices. Then when all devices acknowledge, the
reservation is released. This 'reservation' is related to the
existence of an entry in DRBD's 'transfer hash table'.

If the nbd server receives a write that conflicts with a current
reservation, or if it gets a reservation while it is processing a
conflicting write, it knows there has been a concurrent write.
If it does not detect a conflict, it is still possible that there
were concurrent writes and if so the (or an) other nbd will detect

When conflicting writes are detected, a simple static ordering among
masters determines which write wins. To ensure it's own copy is
valid, the nbd either ignores or applies the second write depending
on the relative priorities of the masters.
To ensure that all other copies are also valid, nbd returns a status
to each writer reporting the collision and whether the write was
accepted or not.

If the raid1 is told that a write collided but was successful, it
must write it out again to any other device that did not detect and
resolve the collision,

Note that this algorithm is somewhat different to the one used by
DRBD. The most obvious difference is that this algorithm sometimes
requires the block to be written twice. DRBD doesn't require that.
DRBD manages differently because the equivalents of the nbd servers can
talk to each other, and see all traffic in both directions. A key
simplification in my model is that they don't. The RAID1 is the only
thing that communicates to an nbd, so any inter-nbd communication
must go through it.
This architectural feature of DRBD is quite possibly the
nail-in-the-coffin of the idea of implementing DRBD inside md/raid1.
I wouldn't be surprised if it is also a feature that would be very
hard to generalise to N nodes.
(Or maybe I just haven't thought hard enough about it.. that's

Bitmap Maintenance

To maintain the bitmap the nbd again needs help from the raid1.
When a write request is submitted to less than the full complement of
targets, the write request carries a 'degraded' flag. Whenever nbd
sees that degraded flag, it sets the bitmap bit for all relevant
sections of the device.
If it sees a write without the 'degraded' flag, it clears the
relevant bits.
Further, if raid1 submits a write to all drives, but some of them
fail, the other drives must be told that the write failed so they can
set the relevant bits. So some sort of "set these bits" message from
the raid1 to the nbd server is needed.

The nbd does not write bitmap updates to storage synchronously.
Rather, it can be told when to flush out ranges of the bitmap. This
is done as part of the RAID1 maintaining it's own record of active

The bitmaps could conceivably be maintained at the RAID1 end and
communicated to the nbd by simple reads and writes. The nbd would
then merge all the bitmaps with a logical 'or'. This would require
more network bandwidth and would require each master to clear bits as
regions were resynced. As such it isn't really a good fit for DRBD.
I mention it only because it is more like the approach currently used
in md.

The enhanced RAID1

As mentioned, the RAID1 in this model sends IO request to 2 (or more)
enhanced nbd device.
Typically one of these will be preferred for reads (in md
terminology, the others are 'write-mostly'). Also the raid1 can
report success for a write before all the nbds have reported success
(write-behind in md terminology).

The raid1 keeps a record of what areas of the device are currently
undergoing IO. This is the activity log in DRBD terminology, or the
write-intent-bitmap in md terminology (though the md bitmap blends
the concepts of the RAID1 level bitmap and the nbd level bitmap).

Before removing a region from this record, the RAID1 tells all nbds
to flush their bitmaps for that region.

Note that this RAID1 level log must be replicated on at least N-1
nodes (where there are N nodes in the system). For the simple case
of N=2, the log can be kept locally (if the local device is working).
For the more general case it needs to be replicated to every device.
In that case it is effectively an addendum to the already-local bitmap.

Other functionality that the RAID1 must implement that has no
equivalent in md and that hasn't been mentioned in the context of
the nbd includes:

- when in a write-behind mode, the raid1 must try to intuit
write-after-write dependencies and generate barrier requests
to enforce them on the write-behind devices.
To do this we have a 'writing' flag.
When a write request arrives, if the 'writing' flag is clear, we
set it and send a write barrier. Then send the write.
When a write completes, we clear the 'writing' flag.

This is not needed in fully synchronous mode as any real
dependency will be imposed by the filesystem on to all devices.


Given the multi-master aspects of DRBD there are interesting
questions about what to do after a crash or network separation -
in particular which device should be treated as the primary.
I'm going treat these as "somebody else's problem". i.e. they are
policy questions that should be handled by some user-space tool.

All I am interested in here is the implementation of the
policy. i.e. how to bring two divergent devices back in to sync.

The basic process is that some thread (and it could conceivably be a
separate 'master') loads the bitmap for one device and then:
if it is the 'primary' device for the resync, it reads all the blocks
mentioned in the bitmap and writes them to all other devices.
if it is not the 'primary' device, it reads all the blocks from the
primary and writes them to the device which owned the bitmap

There is room for some optimisations here to avoid network traffic.
The copying process can request just a checksum from each device and
only copy the data if the checksum differs, or it could load the
checksum from the target of the copy, and then send the source "read
this block only if the checksum is different to X".

The above process would involve a separate resync process for each
device. It would probably be best to perform these sequentially.
An alternate would be to have a single process that loaded all the
bitmaps, merged them and then copied from the primary to all
secondaries for each block in the combined bitmap.
If there were just two nodes and this process always ran on a
specific node - e.g. the non-primary, then this would probably be a
lot simpler than the general solution.

With md, resync IO and normal writes each get exclusive access to the
devices in turn. So writes are blocked while the resync process reads
a few block and writes those blocks.

In the DRBD model where we have more intelligence in the enhanced nbd
this synchronisation can be more finely grained.

The 'reserve' request mentioned above under 'concurrent writes' could
be used, with the resync process given the lowest possible priority
so its write requests always lost if there was a conflict.
Then the resync process would
- reserve an address on the destination (secondary)
- read the block from the primary
- write the block to the destination

Providing that the primary blocked the read while there was a
conflicting write reservation, this should work perfectly.


The list of requests that would be needed to be supported by the
link to the nbd daemon would be something like:
Each of these have sector offset and size
These have no sector/size

RESERVE and SET_BIT could possibly be combined with a WRITE, but
would need to be stand-alone as well.

The extra functionality needed in the RAID1 that has no equivalent
in md/raid1 would be:
- issues RESERVE/RELEASE around write requests
- detecting possible locations for write-barriers when in
write-behind mode
- separate 2-level bitmaps, and other subtleties in
bitmap/activity log handling.
- checksum based resync
- respond to write-conflict errors be re-writing the data block.

Looked at this way, the most complex part would be all the extra
requests that need to be passed to the nbd client. I guess they
would be sent via an ioctl, though there would be some subtlety in
getting that right.
Implementing the new nbd server should be fairly straight forward.
Adding the md/raid1 functionality would probably not be a major
issue, though some more thought will be needed about bitmaps before I
felt completely comfortable about this.

So the summary of the summary is the implementing similar
functionality to DRBD in a md/raid1+nbd style framework appears
to be quite possible.
However for the reasons mentioned under "concurrent writes", a
protocol-compatible implementation is unlikely to be possible.
That also means that the model is not as close as I would like while
doing a code review, but I suspect it is close enough to help.

Thank you for reading. I found the exercise educational. I hope you
did too. I think I might even be ready to review the DRBD code now :-)

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