[linux-lvm] Unable to un-cache logical volume when chunk size is over 1MiB
ryan at magenta.tv
Wed Jun 20 11:10:02 UTC 2018
Kernel is: Linux 3.10.0-693.21.1.el7.x86_64
Distro is: Centos 7 - Linux release 7.4.1708
Zdenek Kabelac wrote on 20/06/2018 11:15:
> Dne 20.6.2018 v 11:18 Ryan Launchbury napsal(a):
>> 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
>> 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
>> 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.
>> 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.
> 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)
Nope, no write errors in /var/log/dmesg. The last log entry was at
10.871493 and the system has been on for 61 days.
More information about the linux-lvm