[linux-lvm] lvmcache writethrough synchronisation at system startup

Zdenek Kabelac zkabelac at redhat.com
Wed Dec 5 14:23:21 UTC 2018

Dne 05. 12. 18 v 7:19 Marcin Wolcendorf napsal(a):
> 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
> the
> 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.


I'd say in your case it's very important which version of kernel is in use, as 
there very couple fixes solving this issue in not too long history.

Secondary issue is - it's always good idea there is correct shutdown process
instead of just 'power-off' althouhg in 'writethrough' case it should not be 
that big issue (but here applies the advice number one - use recent kernels).



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