[Twg] T10 End-to-End Data Integrity HLD

Tue Apr 17 18:02:04 UTC 2012

On Apr 17, 2012, at 12:06 AM, Andreas Dilger wrote:

	On 2012-04-13, at 4:26 PM, Nathan Rutman wrote:

		On Apr 11, 2012, at 11:42 PM, Andreas Dilger wrote:

			I think it should be possible to come to a common solution between the T10 design you presented, and the Merkle tree design that was done for HPCS a few years ago.  

		I view this as different solutions to the problem, but I see no reason why they can't coexist.

	The difficulty with having two independent, but closely related features that solve the same general problem is that this introduces additional code that needs to be maintained indefinitely for the future.  I think that having separate checksum algorithms is fine, and possibly different transports for the checksums (if that is really needed, more below), but there should definitely only be a single framework for the checksum calculation and in-core storage that varies only by the selection of the algorithm.

As far as Lustre is concerned, I view the T10 method as more of a out-of-band data transfer mechanism.  When you say single framework, you imply that the checksum is performed at the same point in the code path, but I think that is fundamentally different between Merkle tree/ZFS (in core) and T10 (in hardware).

		From a technical point of view, the tradeoff here is between transmitting a small amount of data (4k per 1MB) and recalculating that data on the servers.  Depending on where/how often we re-verify the checksums, we could remove the recalculation from the server entirely.

	That is potentially true, but if the checksums are not verified until you get down to the disk, if there ever is a real checksum error you won't have any idea where the error is being introduced.

That's a question about debugging really.  Just as today many sites run without checksums at all due to server CPU loading, it makes sense to me to make intermediate verifications optional.

	I thought the same thing as John - that the appeal of standardized T10 DIF is that it can be verified all along the IO path to isolate the failing components.

T10-DIX is not about fault isolation; it's about keeping protection information with the data for as long as possible.
>From the http://snia.org/sites/default/files/Data_Integrity_Architectural_Model_v0.26.pdf Section 5: 

	In the T10 Protection Information model both Data Integrity Metadata and actual data move through any communication path as a single unit. 
	One can consider that two logical paths exist, one for data and the other for Data Integrity Metadata. The data path is called the Data Channel and the Data Integrity Metadata path is called the Protection Information Channel.

	5.1 Movement of Protection Information
	An important purpose of this specification is to facilitate extending the Protection Envelope from the host-storage interface to the application endpoint. Without extending the protection envelope, the Protection Information Channel terminates at the host storage interface and only the Data Channel continues to the application. Consequently, a key element of this architecture describes how the Protection Information Channel may be extended to the application endpoint.

		From a philosophical point of view, it's between trusting the software to do the right thing in all cases, versus "my own checksums verified by disk hardware" (and trusting only the disk).  I think the appeal of T10 to many people is exactly that - no matter what happens in the intervening layers, in the end I know my checksums have made it to disk.

	I don't see this argument.  Having the OST verify the checksum from the network does not invalidate the additional checking down at the controller and/or disk layer (if there actually are disks that validate DIF, I'm not sure). 

There are.

	 If the OST is regenerating the individual DIF sector checksums for verification by the hardware based on a per-RPC checksum (which has itself been verified to match the RPC checksum), then there is still end-to-end checksum coverage.

	It would be a strange software bug that generated new DIF checksums that were accepted by the hardware (for possibly corrupt data?) but also generated a valid hashed checksum to match the RPC.  Since the RPC hash is a direct hash of the DIF checksums, the only way I can see this happening (for any reasonable RPC hash function, with a 32-bit or 256-bit RPC hash) is if the 16-bit DIF hash on the corrupt data matched the original value, in which case DIF would have missed the corruption as well...

I can give you a trivial example: bad code in the Merkle tree implementation always generates a checksum of 0xDEADBEEF.  Client computes it, server computes it, hey they match, and individual sector recomputed CRC's can still be correct, but the data do not match.

	I can buy the argument that doing the DIF data checksum on the OST consumes CPU cycles, but I had always imagined that it would do this anyway for data/checksum verification as soon as possible instead of waiting until the IO makes it to the controller.  That would ensure that the data the RPC put into the OST cache is valid.

	Waiting for the controller to be the first to do the verification precludes ever having writeback caching on the OST.  A writeback cached RPC would be completed before the data was verified by the controller, and it is not possible (today, at least) for the OST to force the client to resend just that RPC/data for replay.  Any checksum error caused by network errors would require the OST to reset and start recovery for all RPCs in order to get the client to resend the failed RPC.

I agree - if you're going to store the data in a writeback cache with the assumption that all other buffered copies are unavailable, it becomes a requirement to verify the data integrity when storing it in the cache.  

		Mainly for that latter reason we choose to send the CRCs instead of a function of them.  But as I said, our goal is to introduce enough flexibility that multiple data integrity schemes can be used.

			There is enough flexibility in the current Lustre RPC checksum wire format that both the T10 GRD+REF and the ZFS Fletcher4 checksum could be chosen as leaf checksums, and a tree hash can be used for both to get a single RPC hash, giving 2 or 3 new Lustre wire checksum formats.  Since the server side needs to recompute the checksums anyway, it can regenerated GRD tag from the data, get the REF (offset) values from the RPC niobufs, and then rehash it to compare to the RPC checksum.

			Using a Merkle tree for the RPC checksum also avoids the potential problems as described in section 8.5, since the rather large amount of GRD+REF+APP tags do not actually need to be sent with the RPC, only the hash of them (currently a 32-bit value, but it could be expanded to be a 256-bit value if needed/desired).

			The Merkle tree also has the added benefit that the client does not need to know the sector sizes of the OSTs and handle them separately.  Otherwise, there is either a restriction that the OSTs all have to have drives with the same sector sizes (to avoid the need for the client to compute GRD tags differently for each page in a striped file), or the llite layer will need to poke into the OSC for every page to figure out the sector size.

		This is a good point; I think a reasonable solution would require a common sector size within an OST.

	I hope that it would always to be the case that the disks in a single OST have the same sector size, but my concern was more about different sector sizes between OSTs.  In that case, because the data checksum would be dependent on the sector size on each OST, the client per-page checksums would need to be computed based on the OST on which each page was to be written, which adds complexity into the IO path.

	In some sense this is no worse than having different checksum algorithms on different OSTs, but that would largely be an administrator choice that would otherwise default to using the same checksum algorithm for all OSTs.  In contrast, different sector sizes on different OSTs is nearly inevitable over the next several years if new storage is added to an existing system, as 4kB sector drives arrive on the market and users may have little choice but to buy them.

	In summary, I'm not wholly against the idea of shipping the extra T10 GEN+REF data with the RPCs as you propose, so long as there is a single framework for rest of the checksum code for both the pages and the RPC is shared with existing checksum algorithms.

	Cheers, Andreas

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