Dne 13. 10. 22 v 8:53 Pawan Sharma napsal(a):
adding this to lvm-devel mailing list also.
Regards,
Pawan
------------------------------------------------------------------------------
*From:* Pawan Sharma
*Sent:* Wednesday, October 12, 2022 10:42 PM
*To:* linux-lvm@xxxxxxxxxx <linux-lvm@xxxxxxxxxx>
*Cc:* Mitta Sai Chaithanya <mittas@xxxxxxxxxxxxx>; Kapil Upadhayay
<kupadhayay@xxxxxxxxxxxxx>
*Subject:* LVM2 : performance drop even after deleting the snapshot
Hi Everyone,
We are evaluating lvm2 snapshots and doing performance testing on it. This is
what we are doing :
1. dump some data to lvm2 volume (using fio)
2. take the snapshot
3. delete the snapshot (no IOs anywhere after creating the snapshot)
4. run the fio on lvm2 volume
Here as you can see, we are just creating the snapshot and immediately
deleting it. There are no IOs to the main volume or anywhere. When we run the
fio after this (step 4) and we see around 50% drop in performance with
reference to the number we get in step 1.
It is expected to see a performance drop if there is a snapshot because of the
COW. But here we deleted the snapshot, and it is not referring to any data
also. We should not see any performance drop here.
Could someone please help me understand this behavior. Why are we seeing the
performance drop in this case? It seems like we deleted the snapshot but still
it is not deleted, and we are paying the COW penalty.
System Info:
OS : ubuntu 18.04
Kernel : 5.4.0
# lvm version
LVM version:2.02.176(2) (2017-11-03)
Library version: 1.02.145 (2017-11-03)
Driver version:4.41.0
We also tried on latest ubuntu with newer version of LVM. We got the same
behavior.
Hi
Debugging 5 year old software is likely not going to get lot of attention
from upstream.
So please:
a) reproduce the issue with some recent kernel & lvm2
b) take 'dmsetup table && dmsetup status' before you run every 'fio' test
and present here your result in some form - otherwise we can hardly see what
is the problem.
What should be expected - if you use old/thick snapshots - when you 'drop'
snapshot - you have your original intact LV - so results should mostly match
results before you take the snapshot - but you clearly have to take into
account if you use some 'SSD/NVMe' discarding and other things - so always run
series of tests and average your results.
If you use thin snapshot - that you can get various results depending on your
settings of thin chunks, discard usage.
Also maybe try your benchmark with different filesystems...
Regards
Zdenek
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