From: Zdenek Kabelac <zdenek.kabelac@gmail.com>
To: LVM general discussion and development <linux-lvm@redhat.com>,
Erwin van Londen <erwin@erwinvanlonden.net>
Subject: Re: [linux-lvm] [EXTERNAL] Re: LVM2 : performance drop even after deleting the snapshot
Date: Thu, 20 Oct 2022 20:19:12 +0200 [thread overview]
Message-ID: <2e9e0093-d410-f875-58da-7561392abe44@gmail.com> (raw)
In-Reply-To: <2cb2e4787c7b4c30b67ec0d171f5b1f3aa9a9f71.camel@erwinvanlonden.net>
Dne 17. 10. 22 v 15:41 Erwin van Londen napsal(a):
> From the looks of it the disk, as provisioned out of an Azure pool, is likely
> backed by an enterprise raid array. When you provision the pools with
> discard_passdown the removal of the snapshot will also be pushed down to the
> underlying hypervisor or disk array. You would need to wait till that process
> is completed in order to make any comparisons.
>
> ThinVolGrp-ThinDataLV-tpool: 0 1006632960 thin-pool 1 4878/4145152
> 8325/7864320 - rw discard_passdown queue_if_no_space - 1024
>
> As per man page
>
> --discards passdown|nopassdown|ignore
> Specifies how the device-mapper thin pool layer in the kernel should handle
> discards. ignore causes the thin pool to ignore discards. nopassdown causes the
> thin pool to process discards itself to allow reuse of unneeded extents in the
> thin pool. passdown causes the thin pool to process discards itself (like
> nopassdown) and pass the discards to the underlying device.
>
> Try the same operation after changing the thin volume
>
> lvchange --discards nopassdown VG/ThinPoolLV
Discard here is likely irrelevant - since there will likely no blocks for
discarding.
When the user removes thin LV (which happens to be sharing its block with
some other thin LV (origin -> snapshot)) there is just some metadata update
reducing sharing of blocks with origin thinLV - so nothing to be discard for
data (since snapshot is removed after its creation without any use - only if
the origin would be meanwhile in this short period of time changed
dramatically - then exclusively owned parts of such snapshot may be discarded)
Regards
Zdenek
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next prev parent reply other threads:[~2022-10-20 18:19 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-12 17:12 [linux-lvm] LVM2 : performance drop even after deleting the snapshot Pawan Sharma
2022-10-13 6:53 ` Pawan Sharma
2022-10-13 10:50 ` Zdenek Kabelac
2022-10-14 19:31 ` [linux-lvm] [EXTERNAL] " Mitta Sai Chaithanya
2022-10-17 13:10 ` Zdenek Kabelac
2022-10-17 13:41 ` Erwin van Londen
2022-10-20 18:19 ` Zdenek Kabelac [this message]
2022-10-18 3:33 ` Pawan Sharma
2022-10-18 11:15 ` Zdenek Kabelac
2022-10-14 19:50 ` [linux-lvm] " Roger Heflin
2022-10-14 20:28 ` Roberto Fastec
2022-10-17 5:01 ` Kapil Upadhayay
2022-10-17 15:16 ` Demi Marie Obenour
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