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Date: Mon, 23 Mar 2020 08:26:08 +0000
From: Joe Thornber <thornber@redhat.com>
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Subject: Re: [linux-lvm] when bringing dm-cache online,
 consumes all memory and reboots
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To: LVM general discussion and development <linux-lvm@redhat.com>

On Sun, Mar 22, 2020 at 10:57:35AM -0700, Scott Mcdermott wrote:
> have a 931.5 GibiByte SSD pair in raid1 (mdraid) as cache LV for a
> data LV on 1.8 TebiByte raid1 (mdraid) pair of larger spinning disk.
> these disks are hosted by a small 4GB big.little ARM system
> running4.4.192-rk3399 (armbian 5.98 bionic).  parameters were set
> with: lvconvert --type cache --cachemode writeback --cachepolicy smq
> --cachesettings migration_threshold=10000000

If you crash then the cache assumes all blocks are dirty and performs
a full writeback.  You have set the migration_threshold extremely high
so I think this writeback process is just submitting far too much io at once.

Bring it down to around 2048 and try again.

- Joe