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.
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Danti Gionatan
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