On 04/04/2018 11:50 AM, cooloutac wrote:
On Sunday, April 1, 2018 at 5:45:54 PM UTC-4, Chris Laprise wrote:
On 04/01/2018 03:06 PM, 'awokd' via qubes-users wrote:
On Sun, April 1, 2018 6:15 pm, cooloutac wrote:
Doesn't using trim on encrypted drive lessen security though?

In my opinion, it's so marginal a security decrease it hardly bears
mention. :) There's a link in the trim doc for further reading.

I'd like to remind people: Discard and TRIM are not exactly the same thing.

Commands like 'fstrim' only generate discards, and in a Qubes LVM setup
those discards will logically deallocate blocks in the pool, thus
freeing up space for filesystems.

If you enable 'discard' or 'disktrim' or whatever they call it these
days in crypttab, that will result in discards being converted into
hardware TRIM commands. That is because the crypto layer is the one
closest to the hardware in our configuration.

Does TRIM free up space? No. It helps the SSD maintain top write speeds
and perform more efficient wear-leveling. On some drives, it will also
cause the unallocated data to be wiped-out (according to vendor claims).

Does fstrim free up space? Yes! Because it actually generates discards
and the name is misleading.

IMO, the best thing to do is edit your dom0 /etc/fstab and add 'discard'
to the options for root fs '/'. That will free up space in a usable way
and you won't need to run fstrim.


Chris Laprise, tas...@posteo.net
PGP: BEE2 20C5 356E 764A 73EB  4AB3 1DC4 D106 F07F 1886

according to these qubes doc it says it can hurt performance though if added to 
fstab?  It says to use a systemd or cron job instead.  Also says templates and 
vms already have trim enabled automatically.  it also says if we are using luks 
to add it to crypttab or it won't work?

That doc may need some clarifying. TRIM might hurt performance on older SSDs as they wipe data blocks without caching the commands... but without TRIM the discards are just changing block allocation tables. Also, we're talking about dom0 where few filesystem operations are performed; the domUs already all have discard enabled in fstab.

But is this really nescessary to do in dom0? Will it make any difference if all 
the templates and vms already have discard by default?    And so does that mean 
we make the cron job for dom0  we also have to add discard to crypttab?

The default rc5 dom0 config ran fstrim automatically once per week. IMO, that's plenty of time for a user to do something which will lead to an (unnecessary) out of space condition or at least be misinformed about free space much of the time.

The Qubes code has since been changed to add discard to dom0 fstab during install, but I don't know if that made it to the 4.0 release iso. I don't believe the crypttab setting has changed.

TL;dr using discard is necessary when using thin pools (like its necessary when using sparse disk image files) and relying on fstrim for that means it should be run frequently.

To me, the most interesting question all this raises is how close in speed/performance are virtual disk operations on Thin LVM vs Btrfs since they offer similar features and both incur block allocation processing on two separate layers.


Chris Laprise, tas...@posteo.net
PGP: BEE2 20C5 356E 764A 73EB  4AB3 1DC4 D106 F07F 1886

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