Re: compression disk space saving - what are your results?

[Marc Joliet <marcec@web.de>]

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On Sunday 06 December 2015 04:21:30 Duncan wrote:
>Marc Joliet posted on Sat, 05 Dec 2015 15:11:51 +0100 as excerpted:
>> I do think it's interesting that compression (even with LZO) seems to
>> have offset the extra space wastage caused by autodefrag.
>
>I've seen (I think) you mention that twice now.  Perhaps I'm missing
>something... How does autodefrag trigger space wastage?
>
>What autodefrag does is watch for seriously fragmented files and queue
>them up for later defrag by a worker thread.  How would that waste space?
>
>Unless of course you're talking about breaking reflinks to existing
>snapshots or other (possibly partial) copies of the file.

That is in fact what I was referring to.

>But I'd call
>that wasting space due to the snapshots storing old copies, not due to
>autodefrag keeping the current copy defragmented.  And reflinks are
>saving space by effectively storing parts of two files in the same
>extent, not autodefrag wasting it, as the default on a normal filesystem
>would be separate copies, so that's the zero-point base,

Of course, the default on a normal file system is to not have any snapshots 
between which to reflink ;-) .  Also, autodefrag is not a default mount 
option, so the default on BTRFS is to save space via reflinks, which is undone 
by defragmenting, hence why I see it as autodefrag triggering the waste of 
space.

>and reflinks
>save from it, with autodefrag therefore not changing things from the zero-
>point base.  No snapshots, no reflinks, autodefrag no longer "wastes"
>space, so it's not autodefrag's wastage in the first place, it's the
>other mechanisms' saving space.

To my mind it is the keeping of snapshots and the breaking of reflinks via 
autodefrag that together cause space wastage.  This is coming from the 
perspective that snapshots are *useful* and hence by themselves do not 
constitute wasted space.

>From my viewpoint, anyway.  I'd not ordinarily quibble over it one way or
>the other if that's what you're referring to.  But just in case you had
>something else in mind that I'm not aware of, I'm posting the question.

And the above is my viewpoint :-) .

-- 
Marc Joliet
--
"People who think they know everything really annoy those of us who know we
don't" - Bjarne Stroustrup

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