Re: Any suggestion to deal with slow request?

[Jevon Qiao <scaleqiao-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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Hi Robert,

Thank you for the prompt response.

The OSDs are built on XFS and the drives are Intel SSDs.  Each SSD is 
parted into two partitions, one is for journal, the other is for data. 
There is no alignment issue for the partitions.

When slow request msg is outputted, the workload is quite light on the 
replication OSDs.

    Device:         rrqm/s   wrqm/s     r/s     w/s rMB/s    wMB/s
    avgrq-sz avgqu-sz   await  svctm  %util
    sda               0.00     0.00    0.50   30.00     0.00 0.18   
    12.33     0.00    0.08   0.08   0.25
    sdb               0.00     0.50    0.50   78.00     0.00 0.75   
    19.57     0.09    1.20   0.08   0.60
    sdc               0.00     0.50    0.00   28.00     0.00 0.24   
    17.75     0.01    0.32   0.11   0.30

I benchmarked some OSDs with 'ceph tell osd.x bench'，and learned that 
the throughput for some OSDs(the disk usage is over 60%) is 21MB/s, 
which seems abnormal.

    $ ceph tell osd.24 bench
    { "bytes_written": 1073741824,
       "blocksize": 4194304,
       "bytes_per_sec": "22995975.000000"}

But the throughput for some newly added OSDs can reach 370MB/s. I 
suspect if it is related to the GC of SSD. If so, it might explain why 
it takes such long time to write journal. Any idea?

Another phenomenon that the journal_write is queued in writeq for 3 
seconds, I checked the corresponding process logic in function 
FileJournal::submit_entry() and FileJournal::write_thread_entry(), I did 
not find anything suspicious point.

Thanks,
Jevon
On 8/1/16 00:43, Robert LeBlanc wrote:
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> What is the file system on the OSDs? Anything interesting in
> iostat/atop? What are the drives backing the OSDs? A few more details
> would be helpful.
> - ----------------
> Robert LeBlanc
> PGP Fingerprint 79A2 9CA4 6CC4 45DD A904  C70E E654 3BB2 FA62 B9F1
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>
> On Wed, Jan 6, 2016 at 9:03 PM, Jevon Qiao  wrote:
>> Hi Cephers,
>>
>> We have a Ceph cluster running 0.80.9, which consists of 36 OSDs with 3
>> replicas. Recently, some OSDs keep reporting slow request and the cluster
>> has a performance downgrade.
>>
>>  From the log of one OSD, I observe that all the slow requests are resulted
>> from waiting for the replicas to complete. And the replication OSDs are not
>> always some specific ones but could be any other two OSDs.
>>
>> 2016-01-06 08:17:11.887016 7f175ef25700  0 log [WRN] : slow request 1.162776
>> seconds old, received at 2016-01-06 08:17:11.887092:
>> osd_op(client.13302933.0:839452 rbd_data.c2659c728b0ddb.0000000000000024
>> [stat,set-alloc-hint object_size 16777216 write_size 16777216,write
>> 12099584~8192] 3.abd08522 ack+ondisk+write e4661) v4 currently waiting for
>> subops from 24,31
>>
>> I dumped out the historic Ops of the OSD and noticed the following
>> information:
>> 1) wait about 8 seconds for the replies from the replica OSDs.
>>                      { "time": "2016-01-06 08:17:03.879264",
>>                        "event": "op_applied"},
>>                      { "time": "2016-01-06 08:17:11.684598",
>>                        "event": "sub_op_applied_rec"},
>>                      { "time": "2016-01-06 08:17:11.687016",
>>                        "event": "sub_op_commit_rec"},
>>
>> 2) spend more than 3 seconds in writeq and 2 seconds to write the journal.
>>                    { "time": "2016-01-06 08:19:16.887519",
>>                        "event": "commit_queued_for_journal_write"},
>>                      { "time": "2016-01-06 08:19:20.109339",
>>                        "event": "write_thread_in_journal_buffer"},
>>                      { "time": "2016-01-06 08:19:22.177952",
>>                        "event": "journaled_completion_queued"},
>>
>> Any ideas or suggestions?
>>
>> BTW, I checked the underlying network with iperf, it works fine.
>>
>> Thanks,
>> Jevon

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