[linux-lvm] Testing the new LVM cache feature

[Richard W.M. Jones]

On Thu, May 29, 2014 at 04:34:10PM -0400, Mike Snitzer wrote:
> Try using :
> dmsetup message <cache device> 0 write_promote_adjustment 0
> 
> Documentation/device-mapper/cache-policies.txt says:
> 
> Internally the mq policy maintains a promotion threshold variable.  If
> the hit count of a block not in the cache goes above this threshold it
> gets promoted to the cache.  The read, write and discard promote adjustment
> tunables allow you to tweak the promotion threshold by adding a small
> value based on the io type.  They default to 4, 8 and 1 respectively.
> If you're trying to quickly warm a new cache device you may wish to
> reduce these to encourage promotion.  Remember to switch them back to
> their defaults after the cache fills though.

What would be bad about leaving write_promote_adjustment set at 0 or 1?

Wouldn't that mean that I get a simple LRU policy?  (That's probably
what I want.)

> Also, if you discard the entire cache device (e.g. using blkdiscard)
> before use you could get a big win, especially if you use:
> dmsetup message <cache device> 0 discard_promote_adjustment 0

To be clear, that means I should do:

lvcreate -L 1G -n lv_cache_meta vg_guests /dev/fast
lvcreate -L 229G -n lv_cache vg_guests /dev/fast
lvconvert --type cache-pool --poolmetadata vg_guests/lv_cache_meta vg_guests/lv_cache
blkdiscard /dev/vg_guests/lv_cache
lvconvert --type cache --cachepool vg_guests/lv_cache vg_guests/testoriginlv

Or should I do the blkdiscard earlier?

[On the separate subject of volume groups ...]

Is there a reason why fast and slow devices need to be in the same VG?

I've talked to two other people who found this very confusing.  No one
knew that you could manually place LVs into different PVs, and it's
something of a pain to have to remember to place LVs every time you
create or resize one.  It seems it would be a lot simpler if you could
have the slow PVs in one VG and the fast PVs in another VG.

Rich.

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