[LU-3840] llapi_layout API design discussion

• Type: Bug
• Resolution: Duplicate
• Priority: Major
• Severity: 3
• Assignee: Ned Bass (Inactive)
• Reporter: Ned Bass (Inactive)

Description

Extensions to liblustreapi have been posted for review here.

http://review.whamcloud.com/5302

The goal of the API extensions is to provided a user-friendly interface for interacting with the layout of files in Lustre filesystems, and to hide the wire-protocol details behind an opaque data type.

This issue will provide a forum for potentials users and developers to discuss and critique the design of the proposed API extensions.

Issue Links

• is related to: LU-4665 utils: lfs setstripe to specify OSTs
• is related to: LU-2182 Add llapi_file_get_layout() function in liblustreapi
• is related to: LU-3480 Layout Enhancement

Comments

[Jodi Levi (Inactive)]

Duplicate of LU-2182

[Andreas Dilger]

I would much rather stick with returning -1 and errors in errno than having positive error numbers returned from the functions. Many programs I've seen check "if (rc < 0)" for errors instead of "if (rc == -1)" , so I don't think it makes sense to break these.

[James A Simmons]

So I got feedback from our users and middle ware developers. For our users they only care about lfs setstripe and getstripe working. They will most likely never program the striping api themselves. For the ones that do program the most common case is they test for success which means test for zero and then bomb out the app. They normally could care less about returned error values. What is most important to them is that a zero is returned for all functions on success. For our middle ware guys they also want zero to returned for all cases. Now for the error reporting they varied a bit on opinion. What I did get is they don't like negative errno values. They also were not fans of having to read errno itself after a function call. So they asked why not just return a positive errno instead since 0 is success and something else is failure. Also I found out in the discussion return 0 on success and a positive errno is defined in the POSIX.1c standard.

[Christopher Morrone (Inactive)]

Because it would keep the same API that the rest of liblustreapi has today, and there is no drawback to doing so.

The old API is seriously deficient in its design. Yes, we need to keep it around for some time, but I don't see a great deal of value in maintaining compatibility with poor design. This is our chance to make a clean break and make something that applications can really use.

Virtually every application I've seen checks if (rc < 0) instead of if (rc == -1), so as it is clearly documented in the man pages that the functions return a negative value on error instead of "-1" there shouldn't be any problem.

Granted, that is not uncommon. But the man page usually says:

On error, -1 is returned, and errno is set appropriately.

So it is perhaps not unreasonable to guess there are are also folks out there that are used to explicitly checking for -1. Lustre returning only -1 could be considered the least likely to trip anyone up because it will work for checks of either rc == -1 or rc < 0.

[Andy Nelson]

I fully understand the constraints of backwards compatibility and "can't get rid of that yet" cruft. That was very much the point of my previous comment: I've got to carry around backwards compatibility stuff for the different lustre api's until such time as the new one is available on the oldest machine I have to compile code on. I'm expecting that I can remove the "old API" functionality from my own code in something like 5 years or so, if the "new API" stuff lands in a distributed version right now. And that isn't even the case yet, so the date keeps getting pulled further and further out.

An example of that pain is the lustre_idl.h file, which I have to hack by hand and carry around with my code since it doesn't compile in user space and I need stuff out of there. The example codes in the lustre manual include it for example too. All the more complicated since that file changes in odd ways across different versions and I can't just use a 1.8x lustre_idl.h file on a lustre 2.x installation and expect it to work...

So this is why my emphasis on the "start now" and get the clock ticking. As far as new api format (errno and such discussions), we've gone over that, but here is a reiteration: My position as an applications developer is to have one way to access the error information, and for that way to be the errno setting stuff.