[linux-lvm] lvmcache writethrough synchronisation at system startup

Marcin Wolcendorf antymat+redhat at chelmska.waw.pl
Wed Dec 5 06:19:38 UTC 2018

Hi Everyone,

Recently I have set up a simple lvmcache on my machine. This is a write-through
cache, so in my view it should never be necessary to copy data from the
cachepool to the origin LV. But this is exactly what happens: on system start

lvs -o+cache_policy,cache_settings,cache_mode

displays something like:
  LV     VG   Attr       LSize   Pool     Origin       Data%  Meta%  Move Log Cpy%Sync Convert CachePolicy CacheSettings CacheMode   
  home   main Cwi-aoC---  21.83t [cache1] [home_corig] 99.99  0.55            90.39            smq                       writethrough

and for a next day or two it writes the whole cachepool to the origin. The data
on the cached LV seems fine as long as sha512 is concerned. 

Is this supposed to be that way?

My setup:
I have 2 mdraid devices: one 24T raid6, one 1T raid1, and one separate SSD. All
are LUKS-encrypted, SSD is partitioned into MBR boot partition, encrypted /boot
partition and a big encrypted partition for a PV. The encrypted block devices
(except /boot) are PVs in my VG. The 24T RAID holds the origin LV, the 1T RAID
is made into a cachepool.


On a paper submitted by a physicist colleague:

"This isn't right.  This isn't even wrong."
		-- Wolfgang Pauli
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