Re: dear developers, can we have notdatacow + checksumming, plz?

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

Austin S. Hemmelgarn posted on Mon, 21 Dec 2015 08:36:02 -0500 as
excerpted:

> On 2015-12-16 21:09, Christoph Anton Mitterer wrote:

>> On Tue, 2015-12-15 at 11:00 -0500, Austin S. Hemmelgarn wrote:

>>> nodatacow only [avoids fragmentation] if the file is
>>> pre-allocated, if it isn't, then it still ends up fragmented.

>> Hmm is that "it may end up fragmented" or a "it will definitely? Cause
>> I'd have hoped, that if nothing else had been written in the meantime,
>> btrfs would perhaps try to write next to the already allocated blocks.

> If there are multiple files being written, then there is a relatively
> high probability that they will end up fragmented if they are more than
> about 64k and aren't pre-allocated.

Does the 30-second-by-default commit window (and similarly 30-second-
default dirty-flush-time at the VFS level) modify this at all?  It has 
been my assumption that same-file writes accumulated during this time 
should merge, increasing efficiency and decreasing fragmentation (both 
with and without nocow), tho of course further writes outside this 30-
second window will likely trigger it, if other files have been written in 
parallel or in the mean time.

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