Re: [PATCH] btrfs: shrink the size of btrfs_bio

[David Sterba <dsterba@suse.cz>]

On Tue, Dec 09, 2025 at 06:56:47AM +1030, Qu Wenruo wrote:
> 
> 
> 在 2025/12/9 05:49, David Sterba 写道:
> > On Fri, Dec 05, 2025 at 06:34:30PM +1030, Qu Wenruo wrote:
> >> This is done by:
> >>
> >> - Shrink the size of btrfs_bio::mirror_num
> >>    From 32 bits unsigned int to 8 bits u8.
> > 
> > What is the explanation for this? IIRC the mirror num on raid56 refers
> > to the device index,
> 
> You're right, u8 can not cut the max number of devices for RAID6.
> (RAID5 only has two mirrors, mirror 0 meaning reading from data stripes, 
> mirror 1 means rebuild using other data and P stripe)
> 
> BTRFS_MAX_DEVICES() is around 500 for the default 16K node size, which 
> is already beyond 255.
> 
> Although in the real world it can hardly go that extreme, but without a 
> proper rejection/sanity checks, we can not do the shrink now.
> 
> I'd like to limit the device number to something more realistic.
> Would the device limit of 32 cut for both RAID5 and RAID6?
> (And maybe apply this limit to RAID10/RAID0 too?)
> 
> Or someone would prefer more devices?

I'd rather not add such artificial limit, I find 32 to small anyway.
Using say 200+ devices will likely hit other boundaries like fitting
items into some structures or performance reasons, but this does not
justify setting some data structure to u8/1 byte.

With u16 and 16K devices this sounds future proof enough and we may use
u16 in the sructures to save bytes (although it generates a bit worse
code).