Re: Block size and read-modify-write

[Gionatan Danti <g.danti@assyoma.it>]

Il 03-01-2018 23:59 Dave Chinner ha scritto:
> Yes. But I'm talking about the initial page cache writes in your
> tests, and they were all into unwritten extents. These are the
> writes that had different behaviour in exach test case.

I have some difficulties grasping that. Please note that each test is a 
*while loop* of the "dd" command. So, on the first test (dd with cached 
writes), only the first iteration on the loop should write to an 
unwritten extent; the second, third an so on should overwrite real data 
(as "dd" was issued with "oflag=dsync", which immediately flushes data).

So, why you noted that "they were all into unwritten extents"? Again, I 
am missing something?

> That's an application problem, not a filesystem problem. All the
> filesystem can do is align/size the data extents to match what is
> optimal for the underlying storage (as we do for RAID) and hope
> the application is smart enough to do large, well formed IOs to
> the filesystem.

You are right, I am surely approaching the issue from the wrong end...

> I think you've jumped to entirely the wrong conclusion. We do care
> about it because if you can't convey/control data alignment at the
> filesystem level, then you can't fully optimise IO at the
> application level.

Uhm no, it is my (bad) english which failed...
I fully understand XFS does a wonderful job with regard to data 
alignment.
What I inteded to say is that I understand it is not an XFS problem if 
an application does very small writes rather than a large one.

> The reality is that we've been doing these sorts of data alignment
> optimisations for the last 20 years with XFS and applications using
> direct IO. We care an awful lot about alignment of the filesystem
> structure to the underlying device characteristics because if we
> don't then IO performance is extremely difficult to maximise and/or
> make deterministic.
> 
> However, this is such a complex domain that very, very few people
> have the knowledge and expertise to understand how to take advantage
> of it fully. It's hard even to convey just how complex it is to
> people without a solid knowledge base of filesysystem and storage
> knowledge, as this conversion shows...

True. I really thank you for the time spent on explaining the issue.
Apart that studing the source code, there are any resources I can read 
about these advanced topic?

Regards.

-- 
Danti Gionatan
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