Re: [linux-lvm] [EXTERNAL] Re: LVM2 : performance drop even after deleting the snapshot

[Zdenek Kabelac <zdenek.kabelac@gmail.com>]

Dne 17. 10. 22 v 15:41 Erwin van Londen napsal(a):
>  From the looks of it the disk, as provisioned out of an Azure pool, is likely 
> backed by an enterprise raid array. When you provision the pools with 
>   discard_passdown the removal of the snapshot will also be pushed down to the 
> underlying hypervisor or disk array. You would need to wait till that process 
> is completed in order to make any comparisons.
> 
> ThinVolGrp-ThinDataLV-tpool: 0 1006632960 thin-pool 1 4878/4145152 
> 8325/7864320 - rw discard_passdown queue_if_no_space - 1024
> 
> As per man page
> 
> --discards passdown|nopassdown|ignore
> Specifies how the device-mapper thin pool layer in the kernel should handle 
> discards. ignore causes the thin pool to ignore discards. nopassdown causes the
> thin pool to process discards itself to allow reuse of unneeded extents in the 
> thin pool. passdown causes the thin pool to process discards itself (like
> nopassdown) and pass the discards to the underlying device.
> 
> Try the same operation after changing the thin volume
> 
> lvchange --discards nopassdown VG/ThinPoolLV

Discard here is likely irrelevant - since there will likely no blocks for 
discarding.

When the user removes thin LV  (which happens to be sharing its block with 
some other thin LV  (origin -> snapshot)) there is just some metadata update 
reducing sharing of blocks with origin thinLV - so nothing to be discard for 
data (since snapshot is removed after its creation without any use - only if 
the origin would be meanwhile in this short period of time changed 
dramatically - then exclusively owned parts of such snapshot may be discarded)

Regards

Zdenek

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