Re: Why is dedup inline, not delayed (as opposed to offline)? Explain like I'm five pls.

[Duncan <1i5t5.duncan@cox.net>]

Qu Wenruo posted on Mon, 18 Jan 2016 09:36:49 +0800 as excerpted:

>> dedup'ing data immediately when written to high-write-count data is
>> counter productive because no sooner has it been deduped then it is
>> rendered obsolete by another COW write.
> 
> And it seems that you are not familiar how kernel is caching data for
> filesystem.
> There is already kernel page cache for such case.
> No matter how many times you write, as long as you're doing buffered
> write the the data is not written to disk but cached by kernel, until
> either you triggered a manual sync or memory pressure hits threshold.

Not contradicting in general, but checking my own understanding here...

Doesn't the kernel write cache get synced by timeout as well as memory 
pressure and manual sync, with the timeouts found in
/proc/sys/vm/dirty_*_centisecs, with defaults of 5 seconds background and 
30 seconds higher priority foreground expiry?

Regardless, I agree, the kernel page-cache seriously mitigates the stated 
concerns.

-- 
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"Every nonfree program has a lord, a master --
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