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`.
Attachment:
signature.asc
Description: This is a digitally signed message part.
_______________________________________________ linux-lvm mailing list linux-lvm@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
