Re: O_DIRECT vs BLK_FEAT_STABLE_WRITES, was Re: [PATCH] btrfs: never trust the bio from direct IO

[Qu Wenruo <wqu@suse.com>]

在 2025/10/21 18:18, Christoph Hellwig 写道:
> On Tue, Oct 21, 2025 at 01:47:03PM +1030, Qu Wenruo wrote:
>> Off-topic a little, mind to share the performance drop with PI enabled on
>> XFS?
> 
> If the bandwith of the SSDs get close or exceeds the DRAM bandwith
> buffered I/O can be 50% or less of the direct I/O performance.

In my case, the DRAM is way faster than the SSD (tens of GiB/s vs less 
than 5GiB/s).

> 
>> With this patch I'm able to enable direct IO for inodes with checksums.
>> I thought it would easily improve the performance, but the truth is, it's
>> not that different from buffered IO fall back.
> 
> That's because you still copy data.

Enabling the extra copy for direct IO only drops around 15~20% 
performance, but that's on no csum case.

So far the calculation matches your estimation, but...

> 
>> So I start wondering if it's the checksum itself causing the miserable
>> performance numbers.
> 
> Only indirectly by touching all the cachelines.  But once you copy you
> touch them again.  Especially if not done in small chunks.

As long as I enable checksum verification, even with the bouncing page 
direct IO, the result is not any better than buffered IO fallback, all 
around 10% (not by 10%, at 10%) of the direct IO speed (no matter 
bouncing or not).

Maybe I need to check if the proper hardware accelerated CRC32 is 
utilized...

Thanks,
Qu