Dne 21. 09. 20 v 16:26 Mark Mielke napsal(a):
On Mon, Sep 21, 2020 at 10:14 AM nl6720 <nl6720@xxxxxxxxx
<mailto:nl6720@xxxxxxxxx>> wrote:
I wanted to know why the "issue_discards" setting isn't enabled by
default. Are there any dangers in enabling it or if not is there a
chance of getting the default changed?
Also it's not entirely clear to me if/how "issue_discards" affects thin
pool discard passdown.
Historically, there have been dangers. Even today, there might still be
dangers - although, I believe Linux (and other OS) may disable the feature in
hardware which is known to behave improperly.
If you do research and ensure you are using a good storage drive, there should
not be any problems. I enable issue_discards on all systems I work with at
home and work, and have not encountered any problems. But, I also don't buy
cheap drives with questionable namebrands.
It's pretty common for settings such as these to be more conservative, to
ensure that the users who are willing to accept the risk (no matter how small)
can turn it on as an option, and the users who are unaware or might not have
evaluated the risk, cannot blame the software vendors for losing their data.
In the case of LVM - it's not LVM's fault that some drives might lose your
data when discard is sent. But, users of LVM might blame LVM.
Hi
So let's repeat again - issue_discards has nothing to do with discard
handling of the LV itself.
If the LV is sitting on top of SSD - it will receive discard/TRIM no matter
what is set in this issue_discard setting - it could be 0 or 1 and LV will be
discarded.
Anyone can very easily try - create some small simple LV on top SSD - or
thinLV then create i.e. mkfs.ext4 and use 'fstrim -v'.
With discardable device you will get nice print about discarded region -
then play with issue_discards setting - there will be no change...
Setting issue_discards only matters when you run 'lvremove' and your VG is
maybe on some 'provisioned' storage and you pay for used space - in this case
released extents in VG will receive TRIM - so VG will eat less physical
space on such storage.
So keeping this setting 0 is perfectly fine and allows users to revert many
lvm2 operation if they've made a mistake.
When setting is 1 - most of lvm2 commands have NO way back - once done, all
data is gove - which can be quite fatal if you have i.e. very similar LV names...
No distribution should be setting issue_discards to '1' by default - it should
be always changed by admin so he is aware of consequences.
Regards
Zdenek
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