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

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

[Mikhail Morfikov]

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?

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

[Zdenek Kabelac]

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

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

[Mikhail Morfikov]

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?

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

[Zdenek Kabelac]

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

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

[Mikhail Morfikov]

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.