[dm-crypt] md-raid5+lvm+dm-crypt+kvm: one streaming write starves all reads

Christian Pernegger pernegger at gmail.com
Wed Oct 21 22:22:02 CEST 2009

> Is the advantage still there, when you've only got one core available?

[Note that this isn't pretending to be a benchmark, just a few
ballpark figures based off a single bonnie++ run each. I'll redo it
properly on the weekend.]

Seq. reads:
2 cores: ~140MiB/s
1 core: ~100MiB/s
(single crypt dev: ~100MB/s)

Seq. writes:
2 cores: 90 MiB/s
1 core: 65 MiB/s
(single crypt dev: ~100MB/s)

[run 3x dd if=/dev/zero of=testN bs=3M and then try to do something in
another ssh session]
2 cores: barely impacted, maybe 2s for 'top',  5s on 'ls /'
1 core: bad, ~10s for 'top', 35s for 'ls /'
When a command is executed twice in a row (i. e. before everything's
been thrown out of the cache again) it's instant in both cases.
The difference between '1 core' and 'single crypt dev' is hard to
describe. The former feels slooow but steady, the latter bursty.


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