>>>>> "Zdenek" == Zdenek Kabelac <zkabelac@xxxxxxxxxx> 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 _______________________________________________ linux-lvm mailing list linux-lvm@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
