On 16.11.2016 11:24, Zdenek Kabelac wrote:
Dne 15.11.2016 v 13:38 Teodor Milkov napsal(a):
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.
My first 'guess' in this reported case is - the disk I/O traffic seen
is related to the 'reload' of cached chunks from disk back to cache.
This will happen in the case, there has been unclean cache shutdown.
However what is unclean is - why it slows down boot by hours.
Is the cache too big??
Indeed, cache is quite big – a 800GB SSD, but I found experimentally
that this is the size where I get good cache hit ratios with my >10TB
data volume.
As to the 'reload' vs 'flush' – I think it is flushing, because iirc
iostat showed lots of SSD reading and HDD writing, but I'm not really
sure and need to confirm that.
So, are you saying that in case of unclean shutdown this 'reload' is
inevitable?
How much time it takes obviously depends on the SSD size/speed & HDD
speed, but with 800GB SSD it is reasonable to expect very long boot times.
> Can you provide full logs from 'deactivation' and following activation?
Any hints as to how to collect "full logs from 'deactivation' and
following activation"? It happens early in the Debian boot process (I
think udev does the activation) and I'm not sure how to enable
logging... should I tweak /etc/lvm/lvm.conf?
Best regards,
Teodor Milkov
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