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From: Raman Gupta <rocketraman@gmail.com>
Message-ID: <fa65ab01-440c-99a2-a13b-807d302c14e6@gmail.com>
Date: Thu, 29 Dec 2016 08:52:24 -0500
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Subject: [linux-lvm] Two questions about lvm-cache
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To: linux-lvm@redhat.com

I am trying out the lvmcache functionality. So far, everything looks
great. I have two questions:

1) The man page of lvmcache states "With the --cachemode option, the
cache mode can be set when creating a cache LV, or changed on an
existing cache LV." However, I can't figure out the right command to
change the cache mode on an existing LV.

# lvs -a -o +devices
(see https://paste.fedoraproject.org/514698/83019352/raw/)

# lvs -o cache_mode vg_raid10/lv_var
CacheMode
writethrough

# lvconvert --verbose --cachemode writeback vg_raid10/lv_var
(no output)

# lvs -o cache_mode vg_raid10/lv_var
CacheMode
writethrough

# lvconvert --verbose --type cache \
 --cachepool vg_raid10/lv_cache_pool_var \
 --cachemode writeback vg_raid10/lv_var
Executing: /usr/sbin/modprobe dm-cache-mq
Executing: /usr/sbin/modprobe dm-cache-smq
Cache is not supported with cache segment type of the original
logical volume vg_raid10/lv_var.


2) Does lvm manage TRIM on the cache device? Normally I set up SSD
volumes with a weekly fstrim, but that only works for mounted filesystems.


Thanks!
Raman