On 02/01/2024 12.45, Zdenek Kabelac wrote:
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
I see, thanks for the info.
