On 18/05/12 15:04, Lars Ellenberg wrote:
On Wed, May 16, 2012 at 09:11:05PM +0100, Matthew Bloch wrote:
I'm trying to understand a symptom for a client who uses drbd to run
sets of virtual machines between three pairs of servers (v1a/v1b,
v2a/v2b, v3a/v3b), and I wanted to understand a bit better how DRBD I/O
is buffered depending on what mode is chosen, and buffer settings.
Firstly, it surprised me that even in replication mode "A", the system
still seemed limited by by the bandwidth between nodes. I found this
out when the customer's bonded interface had flipped over to its 100Mb
backup connection, and suddenly they had I/O problems. While I was
investigating this and running tests, I noticed that switching to mode A
didn't help, even when measuring short transfers that I'd expect would
fit into reasonable-sized buffers. What kind of buffer size can I
expect from an "auto-tuned" DRBD? It seems important to be able to
cover bursts without leaning on the network, so I'd like to know whether
that's possible with some special tuning.
Uhm, well,
we have invented the DRBD Proxy specifically for that purpose.
That's useful to know - so the kernel buffering, however it's
configured, isn't really set up for handling longer delays? I don't
think that's my problem, as the ICMP ping time between the servers is
<1ms, doesn't drop out even while DRBD reports it hasn't seen its own
pings. It's gigabit ethernet all the way, and on a private LAN.
The other problem is the "PingAck not received" messages that have been
littering the logs of the v3a/v3b servers for the last couple of weeks,
e.g. this has been happening every few hours for one DRBD or another:
May 14 08:21:45 v3b kernel: [661127.869500] block drbd10: PingAck did
not arrive in time.
Increase ping timeout?
I did that (now at 3s, from 0.5s) but I still get reconnections.
I set up a two pairs of VMs to write 1MB to the DRBD every second, and
time it. On the problematic machines, I saw lots of times where the
write took more than 10s, and a couple of those corresponded with DRBD
reconnections. On the normal machines, only two of the writes took more
than 0.1s!
So I'm still hunting for what might be going wrong, even though the
software versions are the same, the drbd links aren't hitting the
ceiling, they're doing no more I/O than the "good" pairs. I think next
will be to take some packet dumps to see if there is anything odd going
on at the TCP layer.
If nobody else on the list has seen this sort of behaviour, and Linbit
have a day rate :-) please get in touch privately, I'd rather get you
guys to fix this for our customer.
Best wishes,
--
Matthew Bloch Bytemark Hosting
http://www.bytemark.co.uk/
tel: +44 (0) 1904 890890
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