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

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

Dne 02. 01. 24 v 12:13 Mikhail Morfikov napsal(a):
> 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?

The whole size of removed LV will be discard.

It's mostly equivalent of using  'blkdiscard /dev/vg/lv' just before calling 
lvremove vg/lv.

lvm2 doesn't interpret in any way what has been used physically used within 
the LV.


Regards

Zdenek