Re: Should issue_discards be enabled when e2scrub is used on an SSD?

[Mikhail Morfikov <mmorfikov@gmail.com>]

On 02/01/2024 11.41, Zdenek Kabelac wrote:
> Dne 29. 12. 23 v 1:48 Mikhail Morfikov napsal(a):
>> I've got the following setup: LUKSv2+LVM on an SSD drive, and I
>> have trim/discad enabled for encrypted container. I also use
>> e2scrub for Online ext4 Metadata Check which uses a 4 GiB LVM
>> snapshot. I currently have 10 logical volumes, which are
>> periodically (once a week) scanned for errors. I've got a question
>> concerning the *issue_discards* option that can be specified in the
>> /etc/lvm/lvm.conf file. What actually happens when this option is
>> enabled and the LVM snapshot is removed after fsck finishes its
>> job? Will the whole 4 GiB be erased on the flash? Is it safe to
>> enable this option in such case, or is it better to leave it 
>> disabled?
> 
> Hello
> 
> issue_discards  is relevant only when you *REMOVE* LVs  - the free
> space in VG  is after such removal 'discarded' - which is only useful
> in case you i.e. pay for 'provisioned' space for your VG (or you have
> some very very very old ssd) - so this will make you VG space
> consumption smaller - but it will also make restoring of accidentally
> removed LVs impossible as the recovered LV will be simply just an
> empty disk space.
> 
> So for 100% you don't need to enabled it for e2scrub as discards are
> normally passed through to your LV device as you may easily check in
> your sysfs dir:
> 
> /sys/block/XXXX/queue/discard_max_bytes
> 
> Regards
> 
> Zdenek
> 

I understand, but the question concerns rather the amount of data written
to the SSD disk. So I have a 4 GiB LVM snapshot, which will be removed
after fsck. Will the whole 4 GiB be written to the disk when issue_discards
is set in the /etc/lvm/lvm.conf file?