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
