Re: Very slow checkpoints

Hi,

On Wed, Mar 18, 2015 at 12:21 PM, Steven Jones
<steven(dot)jones1201(at)outlook(dot)com> 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.