On Tuesday, 22 September 2020 12:14:19 EEST nl6720 wrote: > 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? > Oops, rereading it, lvreduce is actually mentioned in /etc/lvm/lvm.conf. Well, the question about lvconvert and thin pool still stands. > 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/
