[systemd-devel] Authenticated Boot: dm-integrity modes

[Wol]

On 03/12/2021 00:05, Adrian Vovk wrote:
>> The only thing I don't understand is why layering dm-integrity in a loop
> device on top of dm-integrity on a real disk should necessarily hammer
> write performance. I can understand it chewing up ram cache and cpu, but
> it shouldn't magnify real writes that much.
> 
> Well a write in the mounted home dir = 2 writes to the loopback file,
> and a write to the loopback file is 2 writes to the block device. Thus
> a write to the home dir is 4 writes to the block device. Am I
> mistaken?

No I'd say you're right. But if it's a personal system, like my home 
server, I'd be more worried about read speed. Certainly on my system 
(did I say I had raid-5 :-) it's boot times (configuring lvm) and 
reading from disk that I notice.

Given that I have 16GB ram (with 32GB waiting to be installed :-) and 
aren't hammering the system, four (or more) writes via write 
amplification for every write I send from my app is almost un-noticed 
(apart from xosview telling me my cpu cores are working hard). And if I 
have a short burst of writes, there's plenty of cache so as pressure on 
the i/o path goes up it makes elevator optimisation easier and increases 
disk efficiency.

And if I was using a VM on a big server with lots of VMs, again making 
plenty of ram available to cache it smooths out the writes and reduces 
actual pressure on the real physical disks.

Cheers,
Wol