Re: [BUG?] bcachefs performance: read is way too slow when a file has no overwrite.

Kent Overstreet
2024-09-21T16:12:14Z

On Sun, Sep 22, 2024 at 12:02:07AM GMT, David Wang wrote:
> Hi,
>
> At 2024-09-09 21:37:35, "Kent Overstreet" <kent.overstreet@linux.dev> wrote:
> >On Sat, Sep 07, 2024 at 06:34:37PM GMT, David Wang wrote:
>
> >
> >Big standard deviation (high tail latency?) is something we'd want to
> >track down. There's a bunch of time_stats in sysfs, but they're mostly
> >for the write paths. If you're trying to identify where the latencies
> >are coming from, we can look at adding some new time stats to isolate.
>
> About performance, I have a theory based on some observation I made recently:
> When user space app make a 4k(8 sectors) direct write,
> bcachefs would initiate a write request of ~11 sectors, including the checksum data, right?
> This may not be a good offset+size pattern of block layer for performance.
> (I did get a very-very bad performance on ext4 if write with 5K size.)

The checksum isn't inline with the data, it's stored with the pointer -
so if you're seeing 11 sector writes, something really odd is going
on...

I would suggest doing some testing with data checksums off first, to
isolate the issue; then it sounds like that IO pattern needs to be
looked at.

Check the extents btree in debugfs as well, to make sure the extents are
getting written out as you think they are.