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

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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?

Thanks,
Qu

> the type width should not be too narrow. But if
> the semantics are different we can reduce the mirror num in other
> structures as well.
>