Re: Very slow checkpoints

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Hi,

On Wed, Mar 18, 2015 at 12:21 PM, Steven Jones
<steven.jones1201@xxxxxxxxxxx> wrote:
> Hi,

> iostat -x snapshot:
>
> avg-cpu:  %user   %nice %system %iowait  %steal   %idle
>            0.50    0.00    2.35   15.09    0.00   82.05
>
> Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz
> avgqu-sz   await r_await w_await  svctm  %util
> sda               0.00     0.00    0.00    5.00     0.00  2056.00   822.40
> 0.00    0.00    0.00    0.00   0.00   0.00
> sdb               0.00     0.00 1055.00  549.00 41166.50 22840.00    79.81
> 5.28    3.28    4.94    0.10   0.62 100.00
Your sdb is saturated...


> ---
> iotop snapshot:
> Total DISK READ:      41.63 M/s | Total DISK WRITE:      31.43 M/s
>   TID  PRIO  USER     DISK READ  DISK WRITE  SWAPIN     IO>    COMMAND
> 32101 be/4 postgres   10.25 M/s 1085.86 K/s  0.00 % 96.80 % postgres:
> checkpointer process
> 56661 be/4 postgres    6.84 M/s  591.61 K/s  0.00 % 90.91 % postgres:
> dbauser db [local] COPY

> 32102 be/4 postgres    8.05 M/s 1340.47 K/s  0.00 % 82.47 % postgres: writer
> process
>  1005 be/0 root        0.00 B/s    0.00 B/s  0.00 %  5.81 % [txg_sync]
> 32103 be/4 postgres    0.00 B/s   10.41 M/s  0.00 %  0.52 % postgres: wal
> writer process

Why are checkpointer process and writer process reading at 18 MB/s ?
I have no experience with zfs but could it be related to COW and
recordsize? I have no idea if these reads are counted in iotop output
though.


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