[dm-devel] dm-cache issue

[Zdenek Kabelac]

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??

Regards


Zdenek