On Mon, Oct 09, 2023 at 09:27:56AM -0700, Darrick J. Wong wrote: > > + /* > > + * If xfs_bmapi_write finds a delalloc extent at the requested > > + * range, it tries to convert the entire delalloc extent to a > > + * real allocation. > > + * But if the allocator then can't find an AG with enough space > > + * to at least cover the start block of the requested range, > > Hmm. Given that you said this was done in the context of delalloc on > the realtime volume, I don't think there are AGs in play here? Unless > the AG actually ran out of space allocating a bmbt block? Well, really rt group as it was using your rt group patches. Just using AG as the more generalized case here. > My hunch here is that free space on the rt volume is fragmented, but > there were still enough free rtextents to create a large delalloc > reservation. Conversion of the reservation to an unwritten extent > managed to map one free rtextent into the file, but not enough to > convert the file mapping all the way to @startoffset_fsb. Hence the > bmapi_write call succeeds, but returns @nmaps == 0. Yes. > If that's true, I suggest changing the second sentence of the comment to > read: > > "If the allocator cannot find a single free extent large enough to > cover the start block of the requested range, xfs_bmapi_write will > return 0 but leave *nimaps set to 0." That's probably better indeed. I'll wait a bit for more comment and will resend with that update.
