From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mimecast-mx02.redhat.com (mimecast06.extmail.prod.ext.rdu2.redhat.com [10.11.55.22]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 31E1CB34F1 for ; Tue, 22 Sep 2020 09:14:27 +0000 (UTC) Received: from us-smtp-1.mimecast.com (us-smtp-1.mimecast.com [205.139.110.61]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-SHA384 (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 134B718AE948 for ; Tue, 22 Sep 2020 09:14:27 +0000 (UTC) From: nl6720 Date: Tue, 22 Sep 2020 12:14:19 +0300 Message-ID: <2262429.nQIR6vq4BO@walnut> In-Reply-To: <02099334-08df-112d-1d02-dd76cc5d8f65@redhat.com> References: <4972578.pTZnC5dHRY@walnut> <02099334-08df-112d-1d02-dd76cc5d8f65@redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; boundary="nextPart2141521.gE8f6Z8B7l"; micalg="pgp-sha512"; protocol="application/pgp-signature" Subject: Re: [linux-lvm] Why isn't issue_discards enabled by default? Reply-To: LVM general discussion and development List-Id: LVM general discussion and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , List-Id: To: LVM general discussion and development , Zdenek Kabelac --nextPart2141521.gE8f6Z8B7l Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="utf-8"; protected-headers="v1" From: nl6720 To: LVM general discussion and development , Zdenek Kabelac Subject: Re: Why isn't issue_discards enabled by default? Date: Tue, 22 Sep 2020 12:14:19 +0300 Message-ID: <2262429.nQIR6vq4BO@walnut> In-Reply-To: <02099334-08df-112d-1d02-dd76cc5d8f65@redhat.com> References: <4972578.pTZnC5dHRY@walnut> <02099334-08df-112d-1d02-dd76cc5d8f65@redhat.com> 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`. --nextPart2141521.gE8f6Z8B7l Content-Type: application/pgp-signature; name="signature.asc" Content-Description: This is a digitally signed message part. Content-Transfer-Encoding: 7Bit -----BEGIN PGP SIGNATURE----- iHUEABYKAB0WIQQJGlXQbhb/L1xVy7xc6IU14YjTaQUCX2nAawAKCRBc6IU14YjT aT8eAQDR1nBaEtUCfWuR+uePCOJUwMYmeJDf3ToBy156nM8mHgD9FRiTu3suN8M1 G5UL1IHETqcKUZObu7ZFWqkJqssnXAI= =vgGn -----END PGP SIGNATURE----- --nextPart2141521.gE8f6Z8B7l--