[linux-lvm] Unable to un-cache logical volume when chunk size is over 1MiB

[Zdenek Kabelac]

Dne 20.6.2018 v 11:18 Ryan Launchbury napsal(a):
> Hello,
> 
> I'm having a problem uncaching logical volumes when the cache data chunck size 
> is over 1MiB.
> The process I'm using to uncache is: lvconvert --uncache vg/lv
> 
> 
> The issue occurs across multiple systems with different hardware and different 
> versions of LVM.
> 
> Steps to reproduce:
> 
>  1. Create origin VG & LV
>  2. Add cache device over 1TB to the origin VG
>  3. Create the cache data lv:
>     lvcreate -n cachedata -L 1770GB cached_vg /dev/nvme0n1
>  4. Create the cache metadata lv:
>     lvcreate -n cachemeta -L 1770MB cached_vg /dev/nvme0n1
>  5. Convert to a cache pool:
>     lvconvert --type cache-pool --cachemode writethrough --poolmetadata
>     cached_vg/cachemeta cached_vg/cachedata
>  6. Enable caching on the origin LVM:
>     lvconvert --type cache --cachepool cached_vg/cachedata cached_vg/filestore01
>  7. Write some data to the main LV so as the cache device is used:
>     dd if=/dev/zero of=/mnt/filestore01/test.dat bs=1M count=10000
>  8. Check the cache stats:
>     lvs -a -o +cache_total_blocks,cache_used_blocks,cache_dirty_blocks
>  9. Repeating step 8 over time will show that the dirty blocks are not being
>     written back at all
> 10. Try to uncache the device:
>     lvconvert --uncache cached_vg/filestore01
> 11. You will get a repeating message. This will loop indefinitely and not
>     decrease or complete:
>     Flushing x blocks for cache cached_vg/filestore01.
> 
> After testing multiple times, the issue seems to be tied to the chunk size 
> selected in step 5. The LVM man page mentions that the chunk must be a 
> multiple of 32KiB, however the next chunk size automatically assigned over 
> 1MiB is usually 1.03MiB. With a chunk size of 1.03MiB or higher, the cache is 
> not able to flush. Creating a cache device with a chunk size of 1MiB or less, 
> the cache is flushable.
> 
> Now knowing how to avoid the issue, I just need to be able to safely un-cache 
> systems with do have a cache that will not flush.
> 
> Details:
> 
> Version info from lvm version:
> 
> LVM version:     2.02.171(2)-RHEL7 (2017-05-03)
>    Library version: 1.02.140-RHEL7 (2017-05-03)
>    Driver version:  4.35.0

What is the kernel version and Linux distro in use ?

> 
> System info:
> System 1,2,3:
> - Dell R730XD server
> - 12x disk in RAID 6 to onboard PERC/Megaraid controller
> 
> System 4:
> -Dell R630 server
> -60x Disk (6 luns) in RAID 6 to PCI megaraid controller
> 
> The systems are currently in production, so it's quite hard for me to change 
> the configuration to enable logging.
> 
> Any assistance would be much appreciated! If any more info is needed please 
> let me know.

Hi

Aren't there any kernel write errors in your 'dmegs'.
LV becomes fragile if the associated devices with cache are having HW issues 
(disk read/write errors)

Zdenek