On 16.11.2016 16:06, Zdenek Kabelac wrote:
Dne 16.11.2016 v 14:45 Teodor Milkov napsal(a):
On 16.11.2016 11:24, Zdenek Kabelac wrote:
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.
Yep - that's the current trouble of existing dm-cache target.
It's getting inefficient when maintaining more then 1 million
cache block entries - recent versions of lvm2 even do not allow
create such cache without enforcing it.
(so for 32k blocks it' ~30G cache data size)
I'm sorry for not being clear: similarly to the OP my SSD is split among
10 LVs, so eache cache is around 80GB.
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?
Yes - clean shutdown is mandatory - otherwise cache can't know consitency
and has to refresh itself. Other option would be probably to drop cache
and let it rebuild - but you lose already gained 'knowledge' this way.
Anyway AFAIK there is ongoing devel and up-streaming process for new
cache target which will others couple shortcomings and should perform
much
better. lvm2 will supposedly handle transition to a new format in
some way
later.
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?
All you need to collect is basically 'serial' console log from your
machine - so if you have other box to trap serial console log - it's
the most easiest option.
But since you already said you use ~30times bigger cache size then
the size with 'reasonable' performance - I think it's already clear
where is your
problem hidden.
Until new target will be deployed - please consider to use
significantly smaller cache size so the number of cache chunks is not
above 1 000 000.
Thank you very much for your help! I'll give it another go at debugging
what the problem is.
I found dm-writeboost in write_around_mode (kinda write-through) works
well for me, so if I don't manage to get along with dm-cache I have plan B.
Best regards,
Teodor
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