Re: [PATCH v5 00/10] preallocate filter

[Vladimir Sementsov-Ogievskiy]

21.08.2020 17:11, Vladimir Sementsov-Ogievskiy wrote:
> Hi all!
>
> Here is a filter, which does preallocation on write.
>
> In Virtuozzo we have to deal with some custom distributed storage
> solution, where allocation is relatively expensive operation. We have to
> workaround it in Qemu, so here is a new filter.

I have a problem now with this thing.

We need preallocation. But we don't want to explicitly specify it in all the 
management tools. So it should be inserted by default. It's OK for us to keep 
this default different from upstream... But there are problems with the 
implicitly inserted filter (actually iotests fail and I failed to fix them)

1. I have to set bs->inherits_from for filter and it's child by hand after 
bdrv_replace_node(), otherwise bdrv_check_perm doesn't work.

2. I have to set filter_bs->implicit and teach bdrv_refresh_filename() to ignore 
implicit filters when it checks for drv->bdrv_file_open, to avoid appearing of 
json in backing file names

3. And the real design problem, which seems impossible to fix: reopen is 
broken, just because user is not prepared to the fact that file child is a 
filter, not a file node and has another options, and don't support options of 
file-posix.

And seems all it (and mostly [3]) shows that implicitly inserting the filter is 
near to be impossible..

So, what are possible solutions?

In virtuozzo7 we have preallocation feature done inside qcow2 driver. This is 
very uncomfortable: we should to handle each possible over-EOF write to 
underlying node (to keep data_end in sync to be able to shrink preallocation on 
close()).. I don't like this way and don't want to port it..

Another option is implementing preallocation inside file-posix driver. Then, 
instead of BDRV_REQ_NO_WAIT flag I'll need to extend serialising requests API 
(bdrv_make_request_serialising() is already used in file-posix.c) to dupport 
no-wait behavior + expanding the serialising request bounds. This option seems 
feasible, so I'll try this way if no other ideas.

Filter is obviously the true way: we use generic block layer for native request 
serialising, don't need to catch every write in qcow2 driver, don't need to 
modify any other driver and get a universal thing. But how to insert it 
implicitly (or at least automatically in some cases) and avoid all the problems?

--
Best regards,
Vladimir