On 27.08.20 23:08, Vladimir Sementsov-Ogievskiy wrote:
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.
Why?
So it should be inserted by default.
Why? You mean without any option? That seems... Interesting?
(Also like a recipe for reports of performance regression in some cases.)
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)
I would suspect even if the iotests passed we would end up with a heap
of problems that we would only notice at some later point. I thought
you too weren’t too fond of the idea of implicit filters.
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.
Well, what should I say. I feel like we have made efforts in the past
years to make the block graph fully visible to users and yield the
responsibility of managing it to the users, too, so I’m not surprised if
a step backwards breaks that.
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.
Possible, but you haven’t yet explained what the problem with the
management layer inserting the preallocation filter is.
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?
I don’t understand why inserting it implicitly is important.