Re: [pve-devel] [PATCH pve-storage] lvm: use blkdiscard instead cstream to saferemove drive

["DERUMIER, Alexandre via pve-devel" <pve-devel@lists.proxmox.com>]

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From: "DERUMIER, Alexandre" <alexandre.derumier@groupe-cyllene.com>
To: "w.bumiller@proxmox.com" <w.bumiller@proxmox.com>
Cc: "pve-devel@lists.proxmox.com" <pve-devel@lists.proxmox.com>
Subject: Re: [pve-devel] [PATCH pve-storage] lvm: use blkdiscard instead cstream to saferemove drive
Date: Thu, 14 Aug 2025 13:11:00 +0000
Message-ID: <89d59912c82b9b98e1422786aad88a1606108c96.camel@groupe-cyllene.com>

-------- Message initial --------
De: Wolfgang Bumiller <w.bumiller@proxmox.com>
À: "DERUMIER, Alexandre" <alexandre.derumier@groupe-cyllene.com>
Cc: pve-devel@lists.proxmox.com <pve-devel@lists.proxmox.com>
Objet: Re: [pve-devel] [PATCH pve-storage] lvm: use blkdiscard instead
cstream to saferemove drive
Date: 14/08/2025 14:05:51

On Thu, Aug 14, 2025 at 10:07:59AM +0000, DERUMIER, Alexandre wrote:
> > +        saferemove_discard => {
> > +            description => "Wipe with discard instead zeroing.",
> > +            type => 'boolean',
> 
> > > Not sure we need this (not sure when this is actually useful),
> > > but
> > > it's
> > > cheap enough to have around. Should add a `default => 0` for
> > > documentation purposes, though.
> 
> Some storage allow overprovisioning. (create a lun bigger than the
> real
> storage size), it can be interesting to discard instead zeroing (to
> free space on storage side).
> as snapshots currently use lvm volume with same size than the main
> volume, it can be interesting to have overprovisioning.

>>But this does not guarantee that the data is actually erased/zeroed, 
some san array are able to zeroing on discard (but also, some san array
have buggy discard implementation, even if they expose the feature)

>>in
>>which case I'd just disable the thing altogether (but then again I
>>use
>>`issue_discards` in lvm.conf to cause `lvremove` to discard the whole
>>thing... ;-)

yes, this is mostly to avoid the need to tune issue_discards, as if you
have multiple storage, afaik, it's not easy to enable issue_discard for
1 storage and not another.  

I was talking about it with blockbridge guy, I think He's better than
me of this subject :

https://forum.proxmox.com/threads/inside-proxmox-ve-9-san-snapshot-support.169675/
"The main issue we see is ensuring the raw device is fully zeroed
before layering a QCOW on top. QCOW doesn't serialize metadata and data
writes, and it lacks a journal. If the backing device doesn't read
zeros, a power loss or process termination can lead to data corruption.

Addressing this is tricky. Zeroing semantics for unmap/discard vary by
device and implementation, but they can usually be checked via the
device's VPD pages. Offloaded zeroing is worth exploring, though older
systems might not support it, and multiple device vendors have buggy
implementations that the kernel disabled via quirks."


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