Re: [PATCH v2 0/7] large atomic writes for xfs

Christoph Hellwig
2024-12-17T07:11:09Z

On Fri, Dec 13, 2024 at 05:43:09PM +0000, John Garry wrote:
>> So if the redo log uses buffered I/O I can see how that would bloat writes.
>> But then again using buffered I/O for a REDO log seems pretty silly
>> to start with.
>>
>
> Yeah, at the low end, it may make sense to do the 512B write via DIO. But
> OTOH sync'ing many redo log FS blocks at once at the high end can be more
> efficient.
>
> From what I have heard, this was attempted before (using DIO) by some
> vendor, but did not come to much.

I can't see how buffered I/O will be fast than an optimized direct I/O
implementation. Then again compared to very dumb dio code that doesn't
replace the caching in the page I can easily see how dio would be
much worse. But given that people care about optimizing this workload
enough to look into changes all over the kernel I/O stack I would
expected that touching the code to write the redo log should not be
out of the picture.