[linux-lvm] Why isn't issue_discards enabled by default?

[nl6720]

On Monday, 21 September 2020 21:51:39 EEST Zdenek Kabelac wrote:
> Dne 21. 09. 20 v 16:14 nl6720 napsal(a):
> > Hi,
> > 
> > I wanted to know why the "issue_discards" setting isn't enabled by
> > default. Are there any dangers in enabling it or if not is there a
> > chance of getting the default changed?
> > 
> > Also it's not entirely clear to me if/how "issue_discards" affects
> > thin pool discard passdown.
> 
> Hi
> 
> Have you checked it's enclosed documentation in within
> /etc/lvm/lvm.conf ?
> 
> issue_discards is PURELY & ONLY related to sending discard to removed
> disk extents/areas after 'lvremove'.
> 
> It is't not in ANY way related to actual discard handling of the LV
> itself. So if you have LV on SSD it is automatically processing
> discards. From the same reason it's unrelated to discard processing
> of thin-pools.
> 
> And finally why we prefer issue_discards to be disabled (=0) by
> default. It's very simple - with lvm2 we try (when we can) to support
> one-command-back restore - so if you do 'lvremove' - you can use
> vgcfgrestore to restore previous metadata and you have your LV back
> with all the data inside.
> 
> When you have issue_discards=1  - the device gets TRIM - so all the
> data are discarded at device level - so when you try to restore your
> previous metadata - well it's nice - but content is gone forever....
> 
> If user can live with this 'risk' and prefers immediate discard -
> perfectly fine - but it should be (IMHO) admin's  decision.
> 
> Regards
> 
> Zdenek


Thanks for your answer, so the reason it's not enabled by default is to 
allow vgcfgrestore to function.

I have read /etc/lvm/lvm.conf and understand that issue_discards affects 
things like lvremove. But I'd like to know, is it only for lvremove or 
also lvreduce and lvconvert (--merge/--uncache)? And what is its 
relation to thin_pool_discards; with issue_discards = 0 and 
thin_pool_discards = passdown (both the defaults) how far down are the 
discards passed?

Lastly, there's no fstrim equivalent for trimming unused space in a PV, 
right? To do that I'd need to lvcreate a LV occupying all free space and 
then use `lvremove --config devices/issue_discards = 1`.
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