Regarding the mon_osd_min_down_reports I was looking at it recently, this could 
provide some insight


On 10/17/16, 1:36 PM, "ceph-users on behalf of Somnath Roy" 
< on behalf of> wrote:

    Thanks Piotr, Wido for quick response.
    @Wido , yes, I thought of trying with those values but I am seeing in the 
log messages at least 7 osds are reporting failure , so, didn't try. BTW, I 
found default mon_osd_min_down_reporters is 2 , not 1 and latest master is not 
having mon_osd_min_down_reports anymore. Not sure what it is replaced with..
    @Piotr , yes, your PR really helps , thanks !  Regarding each messenger 
needs to respond to HB is confusing, I know each thread has a HB timeout value 
and beyond which it will crash with suicide timeout , are you talking about 
that ?
    -----Original Message-----
    From: Piotr Dałek []
    Sent: Monday, October 17, 2016 12:52 AM
    To:; Somnath Roy;
    Subject: Re: OSDs are flapping and marked down wrongly
    On Mon, Oct 17, 2016 at 07:16:44AM +0000, Somnath Roy wrote:
    > Hi Sage et. al,
    > I know this issue is reported number of times in community and attributed 
to either network issue or unresponsive OSDs.
    > Recently, we are seeing this issue when our all SSD cluster (Jewel based) 
 is stressed with large block size and very high QD. Lowering QD it is working 
just fine.
    > We are seeing the lossy connection message like below and followed by the 
osd marked down by monitor.
    > 2016-10-15 14:30:13.957534 7f6297bff700  0 --
    > submit_message osd_op_reply(1463
    > rbd_data.55246b8b4567.000000000000d633 [set-alloc-hint object_size
    > 4194304 write_size 4194304,write 3932160~262144] v222'95890 uv95890
    > ondisk = 0) v7 remote,, dropping message
    > In the monitor log, I am seeing the osd is reported down by peers and 
subsequently monitor is marking it down.
    > OSDs is rejoining the cluster after detecting it is marked down wrongly 
and rebalancing started. This is hurting performance very badly.
    > My question is the following.
    > 1. I have 40Gb network and I am seeing network is not utilized beyond 
10-12Gb/s , no network error is reported. So, why this lossy connection message 
is coming ? what could go wrong here ? Is it network prioritization issue of 
smaller ping packets ? I tried to gaze ping round time during this and nothing 
seems abnormal.
    > 2. Nothing is saturated on the OSD side , plenty of 
network/memory/cpu/disk is left. So, I doubt my osds are unresponsive but yes 
it is really busy on IO path. Heartbeat is going through separate messenger and 
threads as well, so, busy op threads should not be making heartbeat delayed. 
Increasing osd heartbeat grace is only delaying this phenomenon , but, 
eventually happens after several hours. Anything else we can tune here ?
    There's a bunch of messengers in OSD code, if ANY of them doesn't respond 
to heartbeat messages in reasonable time, it is marked as down. Since packets 
are processed in FIFO/synchronous manner, overloading OSD with large I/O will 
cause it to time-out on at least one messenger.
    There was an idea to have heartbeat messages go in the OOB TCP/IP stream 
and process them asynchronously, but I don't know if that went beyond the idea 
    > 3. What could be the side effect of big grace period ? I understand that 
detecting a faulty osd will be delayed, anything else ?
    Yes - stalled ops. Assume that primary OSD goes down and replicas are still 
alive. Having big grace period will cause all ops going to that OSD to stall 
until that particular OSD is marked down or resumes normal operation.
    > 4. I saw if an OSD is crashed, monitor will detect the down osd almost 
instantaneously and it is not waiting till this grace period. How it is 
distinguishing between unresponsive and crashed osds ? In which scenario this 
heartbeat grace is coming into picture ?
    This is the effect of my PR#8558 (
    which causes any OSD that crash to be immediately marked as down, 
preventing stalled I/Os in most common cases. Grace period is only applied to 
unresponsive OSDs (i.e. temporary packet loss, bad cases of network lags, 
routing issues, in other words, everything that is known to be at least 
possible to resolve by itself in a finite amount of time). OSDs that crash and 
burn won't respond - instead, OS will respond with ECONNREFUSED indicating that 
OSD is not listening and in that case the OSD will be immediately marked down.
    Piotr Dałek
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