On 14.11.2016 17:34, Zdenek Kabelac wrote:
Dne 14.11.2016 v 16:02 Alexander Pashaliyski napsal(a):
The server is booting for hours, because of IO load. It seems is
triggered a flush from SSD disk (that is used for a cache device) to
the raid controllers (they are with slow SATA disks).
I have 10 cached logical volumes in *writethrough mode*, each with 2T
of data over 2 raid controllers. I use a single SSD disk for the cache.
The backup system is with lvm2-2.02.164-1 & kernel 4.4.30.
Do you have any ideas why such flush is triggered? In writethrough
cache mode
we shouldn't have dirty blocks in the cache.
Have you ensured there was proper shutdown ?
Cache needs to be properly deactivated - if it's just turned off,
all metadata are marked dirty.
Zdenek
Hi,
I'm seeing the same behavior described by Alexander. Even if we assume
something is wrong with my shutdown scripts, still how could dm-cache
ever be dirty in writethrough mode? What about the case where server
crashes for whatever reason (kernel bug, power outage, operator error
etc.)? Waiting several hours, or for sufficiently large cache even days
for the system to come back up is not practical.
I found this 2013 conversation, where Heinz Mauelshagen <heinzm redhat
com> states that "in writethrough mode the cache will always be coherent
after a crash":
https://www.redhat.com/archives/dm-devel/2013-July/msg00117.html
I'm thinking for a way to --uncache and recreate cache devices on every
boot, which should be safe in writethrough mode and takes reasonable,
and more importantly – constant amount of time.
Best regards,
Teodor Milkov
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