[linux-lvm] when bringing dm-cache online, consumes all memory and reboots

[John Stoffel]

>>>>> "Zdenek" == Zdenek Kabelac <zkabelac at redhat.com> writes:

Zdenek> Dne 23. 03. 20 v 9:26 Joe Thornber napsal(a):
>> On Sun, Mar 22, 2020 at 10:57:35AM -0700, Scott Mcdermott wrote:
>>> have a 931.5 GibiByte SSD pair in raid1 (mdraid) as cache LV for a
>>> data LV on 1.8 TebiByte raid1 (mdraid) pair of larger spinning disk.
>>> these disks are hosted by a small 4GB big.little ARM system
>>> running4.4.192-rk3399 (armbian 5.98 bionic).  parameters were set
>>> with: lvconvert --type cache --cachemode writeback --cachepolicy smq
>>> --cachesettings migration_threshold=10000000
>> 
>> If you crash then the cache assumes all blocks are dirty and performs
>> a full writeback.  You have set the migration_threshold extremely high
>> so I think this writeback process is just submitting far too much io at once.
>> 
>> Bring it down to around 2048 and try again.
>> 

Zdenek> Hi

Zdenek> Users should be 'performing' some benchmarking about the 'useful' sizes of
Zdenek> hotspot areas - using nearly 1T of cache for 1.8T of origin doesn't look
Zdenek> the right ration for caching.
Zdenek> (i.e. like if your CPU cache would be halve of your DRAM)

Zdenek> Too big 'cache size' leads usually into way too big caching
Zdenek> chunks (since we try to limit number of 'chunks' in cache to 1
Zdenek> milion - you can rise up this limit - but it will consume a
Zdenek> lot of your RAM space as well) So IMHO I'd recommend to use at
Zdenek> most 512K chunks - which gives you about 256GiB of cache size
Zdenek> - but still users should be benchmarking what is the best for
Zdenek> them...)

I think dm-cache should be smarter as well, and not let the users
bring the system to it's knees with outrageous numbers.  When a user
puts a migration_threshold that high, there needs to be a safety check
that the system isn't them using too much memory, and should listen to
memory pressure instead.

Also, can you change the migration_threshold without activating?  Or
when activated?

John