On 2017-02-07 14:31, Peter Zaitsev wrote:
Hi Hugo,
As I re-read it closely (and also other comments in the thread) I know
understand there is a difference how nodatacow works even if snapshot are
in place.
On autodefrag I wonder is there some more detailed documentation about how
autodefrag works.
The manual https://btrfs.wiki.kernel.org/index.php/Mount_options has
very general statement.
What does "detect random IO" really means ? It also talks about
defragmenting the file - is i really about the whole file which is
triggered for defrag or is defrag locally ? Ie I would understand what
as writes happen the 1MB block is checked and if it is more than X
fragments it is defragmented or something like that.
I don't know the exact algorithm, but I'm pretty sure it's similar to
what bcache uses to bypass the cache device for sequential I/O. In
essence, it's going to trigger for database usage.
Also does autodefrag works with nodatacow (ie with snapshot) or are these
exclusive ?
I'm not sure about this one. I would assume based on the fact that many
other things don't work with nodatacow and that regular defrag doesn't
work on files which are currently mapped as executable code that it does
not, but I could be completely wrong about this too.
There's another approach which might be worth testing, which is to
use autodefrag. This will increase data write I/O, because where you
have one or more small writes in a region, it will also read and write
the data in a small neghbourhood around those writes, so the
fragmentation is reduced. This will improve subsequent read
performance.
I could also suggest getting the latest kernel you can -- 16.04 is
already getting on for a year old, and there may be performance
improvements in upstream kernels which affect your workload. There's
an Ubuntu kernel PPA you can use to get the new kernels without too
much pain.
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