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

[Zdenek Kabelac]

Dne 22. 09. 20 v 11:14 nl6720 napsal(a):
> 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

There is currently one known exception - pvmove - which is not trivial to 
resolve.  All other 'removals' go through standard extent release and
can be discarded when wanted (unless we missed some other use-case).

> relation to thin_pool_discards; with issue_discards = 0 and
> thin_pool_discards = passdown (both the defaults) how far down are the
> discards passed?

thin-pool is using LVs  - so this is again about handling the discard on a 
_tdata LV and it is completely unrelated to issue_discards setting.


> 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`.

Well there is one easily 'scriptable'

You can simply allocate the free space in your VG (lvcreate -l100%FREE)
and then simply use  'blkdiscard /dev/vg/my_discardable_lv'
Once finished - release the LV.

We may eventually introduce some 'pollable' support as some discards can take 
extremely long time depending on type of a device.
However at this moment this is not really seen as priority...

Regards

Zdenek