[dm-devel] dm-bufio

Kasatkin, Dmitry dmitry.kasatkin at intel.com
Tue Mar 27 10:20:00 UTC 2012

On Sat, Mar 24, 2012 at 8:51 PM, Will Drewry <redpig at dataspill.org> wrote:
> On Fri, Mar 23, 2012 at 8:07 PM, Mikulas Patocka <mpatocka at redhat.com> wrote:
>> On Sat, 24 Mar 2012, Kasatkin, Dmitry wrote:
>>> Hi,
>>> Thanks for clarification.
>>> Indeed everything works just with dm_bufio_write_dirty_buffers().
>>> Reboot notifier is to issue the flush only..
>>> As I understand, dm-bufio will do the flush but currently once per 10 seconds.
>>> if data on the block device and metadata on other block device get out
>>> of sync, what you can do then?
>>> how journal helps then?
>>> - Dmitry
>> It depends what you're trying to do.
>> If you're trying to do something like "dm-verity", but with a possibility
>> to write to the device, there are several possibilities:
>> * keep two checksums per 512-byte sector, the old checksum and the new
>> checksum. If you update the block, you update the new checksum, sync the
>> metadata device and then write to the data device (obviously you need to
>> batch this update-sync-write for many blocks write concurrently to get
>> decent performance). When you verify block, you allow either checksum to
>> match. When you sync caches on the data device, you must forget all the
>> old checksums.
>> * use journaling, write data block and its checksum to a journal. If the
>> computer crashes, you just replay the journal (so you don't care what data
>> was present at that place, you overwrite it with data from the journal).
>> The downside is that this doubles required i/o throughput, you should have
>> journal and data on different devices.
>> * do nothing and rebuild the checksums in case of crash. It is simplest,
>> but it doesn't protect from data damages that happen due to the crash (for
>> example, some SSDs corrupt its metadata on power failure and you can't
>> detect this if you rebuild checksums after a power failure).
>>> Yes.. I am aware of dm-verity target.
>>> It suites well for read-only cases.
>>> It is questionable how tree-based approach will work with read-write.
>>> Each single update will cause whole tree recalculation.
>> A write would recalculate hashes only in the branch from tree bottom to
>> tree top. The obvious downside is that there is no protection from crash.
> It also depends on how you plan to assure the integrity of the data:
> Device-based symmetric key, asymmetric key, etc and the costs
> associated.  Local updates make integrity tricky -- will the device
> update itself or will signed updates be supplied, do they need to be
> online, does only a subsection need to be online, etc.
> It's likely that the tree updates won't be too expensive compared to
> the crypto and you could attempt to optimize tree updates along a hot
> path if needed (by breaking out hot subdirs to a separate targets) and
> explore other tricks for getting transaction oriented behavior (two
> swapping metadata devices for atomic tree updates, etc).  dm-verity
> was never locked into being a read-only target, but the lack of need
> to support online updates means the code and required changes don't
> exist.
> I'm sure any of us involved in dm-verity would be happy to discuss how
> it might be used for your purposes (or if it is really a bad fit),
> etc.
> cheers!
> will


dm-verity looks extremely nice for read-only targets.
Just one root hash protects whole block device.
But I am interested in writable use case.
Would be nice to understand how it can be addresses as well...


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