Re: [RFC 0/3] block/file-posix: Work around XFS bug

[Alberto Garcia]

On Fri 25 Oct 2019 04:19:30 PM CEST, Max Reitz wrote:
>>> So, it's obvious that c8bb23cbdbe32f5c326 is significant for 1M
>>> cluster-size, even on rotational disk, which means that previous
>>> assumption about calling handle_alloc_space() only for ssd is wrong,
>>> we need smarter heuristics..
>>>
>>> So, I'd prefer (1) or (2).
>
> OK.  I wonder whether that problem would go away with Berto’s subcluster
> series, though.

Catching up with this now. I was told about this last week at the KVM
Forum, but if the problems comes with the use of fallocate() and XFS,
the I don't think subclusters will solve it.

handle_alloc_space() is used to fill a cluster with zeroes when there's
COW, and that happens the same with subclusters, just at the subcluster
level instead of course.

What can happen, if the subcluster size matches the filesystem block
size, is that there's no need for any COW and therefore the bug is never
triggered. But that's not quite the same as a fix :-)

> Maybe make a decision based both on the ratio of data size to COW area
> length (only invoke handle_alloc_space() under a certain threshold),
> and the absolute COW area length (always invoke it above a certain
> threshold, unless the ratio doesn’t allow it)?

Maybe combining that with the smaller clusters/subclusters can work
around the problem. The maximum subcluster size is 64KB (for a 2MB
cluster).

Berto