On 17.08.20 16:32, Kevin Wolf wrote: > Am 17.08.2020 um 15:51 hat Max Reitz geschrieben: >> On 17.08.20 15:13, Kevin Wolf wrote: >>> Am 17.08.2020 um 14:56 hat Max Reitz geschrieben: >>>> On 13.08.20 18:29, Kevin Wolf wrote: >>>>> qemu-nbd allows use of writethrough cache modes, which mean that write >>>>> requests made through NBD will cause a flush before they complete. >>>>> Expose the same functionality in block-export-add. >>>>> >>>>> Signed-off-by: Kevin Wolf >>>>> --- >>>>> qapi/block-export.json | 7 ++++++- >>>>> blockdev-nbd.c | 2 +- >>>>> 2 files changed, 7 insertions(+), 2 deletions(-) >>>>> >>>>> diff --git a/qapi/block-export.json b/qapi/block-export.json >>>>> index 1fdc55c53a..4ce163411f 100644 >>>>> --- a/qapi/block-export.json >>>>> +++ b/qapi/block-export.json >>>>> @@ -167,10 +167,15 @@ >>>>> # Describes a block export, i.e. how single node should be exported on an >>>>> # external interface. >>>>> # >>>>> +# @writethrough: If true, caches are flushed after every write request to the >>>>> +# export before completion is signalled. (since: 5.2; >>>>> +# default: false) >>>>> +# >>>>> # Since: 4.2 >>>>> ## >>>>> { 'union': 'BlockExportOptions', >>>>> - 'base': { 'type': 'BlockExportType' }, >>>>> + 'base': { 'type': 'BlockExportType', >>>>> + '*writethrough': 'bool' }, >>>>> 'discriminator': 'type', >>>>> 'data': { >>>>> 'nbd': 'BlockExportOptionsNbd' >>>> >>>> Hm. I find it weird to have @writethrough in the base but @device in >>>> the specialized class. >>>> >>>> I think everything that will be common to all block exports should be in >>>> the base, and that probably includes a node-name. I’m aware that will >>>> make things more tedious in the code, but perhaps it would be a nicer >>>> interface in the end. Or is the real problem that that would create >>>> problems in the storage daemon’s command line interface, because then >>>> the specialized (legacy) NBD interface would no longer be compatible >>>> with the new generalized block export interface? >>> >>> Indeed. I think patch 15 has what you're looking for. >> >> Great. :) >> >> Discussions where both participants have the same opinion from the >> start are the best ones. > > Makes things a lot easier. > > Maybe I should try to move patch 15 earlier. The series is mostly just > in the order that I wrote things, but there were also a few nasty > dependencies in the part the generalises things from NBD to BlockExport. > So I'm not sure if this is a patch that can be moved. > >>>> Anyway, @writable might be a similar story. A @read-only may make sense >>>> in general, I think. >>> >>> Pulling @writable up is easier than a @read-only, but that's a naming >>> detail. >> >> Sure. >> >>> In general I agree, but this part isn't addressed in this series yet. >>> Part of the reason why this is an RFC was to find out if I should >>> include things like this or if we'll do it when we add another export >>> type or common functionality that needs the same option. >> >> Sure, sure. > > So should I or not? :-) Can we delay it until after this series? I.e., as long as it retains the name “writable”, would pulling it up into BlockExportOptions a compatible change? If so, then I suppose we could do it afterwards. But I think it does make the most sense to “just” do it as part of this series. >> Meta: I personally don’t like RFC patches very much. RFC to me means >> everything is fair game, and reviewers should be free to let their >> thoughts wander and come up with perhaps wild ideas, just trying to >> gauge what everyone thinks. >> >> When I’m the submitter, I tend to get defensive then, because I’ve >> invested time in writing the code already, so I tend to be biased >> against fundamental changes. (Horrible personal trait. I’m working >> on it.) > > This makes sense. Nobody likes having to rewrite their RFC series. > > But there is one thing I dread even more: Polishing the RFC series for > another week until I can send it out as non-RFC and _then_ having to > rewrite it. Yes. Especially bad with tests. >> As a reviewer, the code and thus some fleshed out design is there >> already, so it’s difficult to break free from that and find completely >> different solutions to the original problem. >> (I kind of ventured in that direction for this patch, and it seems like >> you immediately noticed that my response was different from usual and >> pointed out the RFC status, perhaps to make me feel more comfortable in >> questioning the fundamental design more. Which I noticed, hence this >> wall of text.) > > Basically just telling you that I was already interested in your input > for this point specifically when I sent the series. OK :) >> Perhaps I’m wrong. Perhaps it’s just myself (the points I’ve just >> listed are definitely my own personal weaknesses), but I can’t help but >> project and assume that others may feel similar, at least in part. >> So I feel like RFCs that consist of patches tend to at least lock me in >> to the solution that’s present. I find them difficult to handle, both >> as a submitter and as a reviewer. >> >> All in all, that means on either side I tend to handle patch RFCs as >> “Standard series, just tests missing”. Not sure if that’s ideal. Or >> maybe that’s exactly what patch RFCs are? >> >> (Of course, it can and should be argued that even for standard series, I >> shouldn’t be afraid of questioning the fundamental design still. But >> that’s hard...) > > I usually send RFC patches when I know that I wouldn't consider them > mergable yet, but I don't want to invest the time to polish them before > I know that other people agree with the approach and the time won't be > wasted. > >> But, well. The alternative is writing pure design RFCs, and then you >> tend to get weeks of slow discussion, drawing everything out. Which >> isn’t ideal either. Or is that just a baseless prejudice I have? > > In many cases (and I think this is one of them in large parts), I only > really learn what the series will look like when I write it. That’s true. With a pure design RFC, it’s often difficult to know even the scope of the design until you’ve begun to write code. So there’s a danger of just writing a bunch of uncontroversial basic design stuff because one has no idea of what may actually become problematic and questionable. :/ > I could have sent a design RFC for the QAPI part, but I didn't expect > this to be contentious because it's just the normal add/del/query thing > that exists for pretty much everything else, too. Yeah, the functions themselves are clear. Hm. Perhaps software engineering just is actually difficult, and there’s no way around it. >>>> Basically, I think that the export code should be separate from the code >>>> setting up the BlockBackend that should be exported, so all options >>>> regarding that BB should be common; and those options are @node-name, >>>> @writethrough, and @read-only. (And perhaps other things like >>>> @resizable, too, even though that isn’t something to consider for NBD.) >>> >>> Do you mean that the BlockBackend should already be created by the >>> generic block export layer? >> >> It would certainly be nice, if it were feasible, don’t you think? >> >> We don’t have to bend backwards for it, but maybe it would force us to >> bring the natural separation of block device and export parameters to >> the interface. > > I can try. I seem to remember that you had a reason not to do this the > last time we discussed generalised exports, but I'm not sure what it > was. > > The obvious one could be that the block export layer doesn't know which > permissions are needed. But it can always start with minimal permissions > and let the driver do a blk_set_perm() if it needs more. Trying sounds good. Since there shouldn’t be consequences for the QMP interface, we™ can always try again later (i.e., when adding more export types). Max