Re: [PATCH] btrfs: Limit size of bios submitted from writeback

[Qu Wenruo <wqu@suse.com>]

在 2026/4/22 22:19, Jan Kara 写道:
[...]
>> Iterating through all chunk maps may take some time for huge filesystems.
> 
> Yeah, I was wondering a bit. But since this is done only on mount, I
> figured it doesn't matter much.
> 
>> Meanwhile the device list is way smaller than the chunk maps, what about
>> iterating through all devices instead?
> 
> I was thinking about that as well. But as you can see below, we also need
> to know how the devices are put together in a raid and AFAIU that
> information isn't there at the device level... If there's some better data
> structure to iterate to get information about raid configurations of all
> pieces of btrfs filesystem, I can certainly use that.

My idea would be just ignore the detailed RAID configuration completely.

One point is, btrfs can have more than one data profiles (e.g. during 
balance, or canceled balance, or degraded writes), and even there is 
only a single data profile, we can still have different optimized sizes, 
e.g. RAID10 on very unbalanced disks.

Furthermore for real multi-device btrfs setup, it's very likely the data 
profile is at least striped (RAID0/RAID10), thus the huge bio will be 
split into 64K stripes by btrfs before submission already.

It's really affecting single-device profiles like SINGLE/DUP and 
mirror-only profiles (RAID1*), where we directly submit such huge bio to 
each device.

With that said, it will be much easier to ignore the complex RAID 
profile iteration, and just grab the min/max optimal io size of each 
device, multiple it by the number of disks and call it a day.

Thanks,
Qu