[qubes-users] Need to safely reduce lvm volume: Don't want to lose pruned monero blockchain

Haisam_Khalil · June 6, 2021, 3:29pm

Hello all,

Attached is a screenshot of relevant lvs output. The crypto-xmr-throwaway volume is not actually that size, it was originally sized to that to download the monero blockchain via cli and then to allow for a pruned version to be made.

Qubes Version: 4.0 (verified via qubes manager)

Problem: I want lvm space back without bricking the qube and having to restore the qube from a backup. sudo lvs shows that crypto-xmr-throwaway volume are oversized, despite deleting a 105GB file from the qube (yes i checked the trash to be sure).

Attempted solutions: Attempted to reduce VM size (nukes the .bitmonero hidden folder that contains the blockchains; tried backing up the pruned blockchain to veracrypted SSD, then copying said blockchain+assorted files to a new qube (input/output error when trying to copy pruned blockchain back over to new qube),

github.com

Qubes-Community/Contents/blob/master/docs/configuration/disk-trim.md


Disk Trim
=========

Disk trimming is the procedure by which the operating system informs the underlying storage device of which storage blocks are no longer in use.
It does this by issuing an `ATA_TRIM` command for the block. This is also known as a `discard`.
In this way, the storage device can perform garbage collection of the unused blocks and internally prepare them for reuse. SSDs in general benefit from this, while HDDs do not.

In a Linux system running on bare metal, this is relatively straight-forward. 
When instructed by the operating system, discards are issued by the file-system driver directly to the storage driver and then to the SSD.

In Qubes, this gets more complex due to virtualization, LUKS, and LVM (and thin pools on R4.0 and up).
If you run `fstrim --all` inside a TemplateVM, in a worst case the `discard` can follow a path like:

    OS -> File-system Driver -> Virtual Storage Driver -> Backend Storage Driver -> LVM Storage Driver -> LUKS Driver -> Physical Storage Driver -> Physical Storage Device
    
If discards are not supported at any one of those layers, it will not make it to the underlying physical device.

There are some security implications to permitting TRIM (read for example [this article](https://asalor.blogspot.com/2011/08/trim-dm-crypt-problems.html)), but in most cases not exploitable.
Conversely, TRIM can improve security against local forensics when using SSDs, because with TRIM enabled deleting data (usually) results in the actual data being erased quickly, rather than remaining in unallocated space indefinitely.

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Current solution to be attempted: Create a new qube and move the blockchain+monero files over to a new qube and hope it works out.

Ask: How can I reduce or trim the lvm volume crypto-xmr-throwaway to something around 50gb without breaking anything? Alternatively: how can I safely transfer over the pruned blockchain without breaking anything.

I’ll report back if my attempt at fixing works. Otherwise please help me out if you can. Thank you.