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IDS Forum

Re: RE: RE: RE: Informix 12.1 Blocking Checkpoints

Posted By: Andreas Legner1
Date: Wednesday, 8 November 2017, at 6:28 a.m.

In Response To: Re: RE: RE: RE: Informix 12.1 Blocking Checkpoints (NATYURAL HORACIO)

From 'onstat -g ckp' there appear to be two problems which might be related
(or one exaggerated by the other), but still should count separately:

I tried to format a little more readable, hopefully this remains:

Clock Total Flush Block # Ckpt
Wait Long # Dirty Dskflu Total Avg Total Avg
Interval Time Trigger LSN Time Time Time Waits Time
Time Time Buffers /Sec Pages /Sec Pages /Sec
1027271 15:58:17 CKPTINTVL 1477179:0x7de2018 20.7 17.7 0.0 18 0.0
1.2 2.5 34765 1961 6889 22 19702 64
1027272 16:04:14 CKPTINTVL 1477180:0x28ca018 46.3 44.5 0.0 7 0.0
0.9 1.3 19817 445 6706 20 20341 61
1027273 16:08:48 CKPTINTVL 1477180:0x75242e0 4.4 3.6 0.0 6 0.0
0.3 0.7 14482 4069 7140 22 19877 63
1027274 16:13:53 CKPTINTVL 1477180:0xc44d018 5.8 5.0 0.0 7 0.0
0.4 0.5 16719 3321 7465 24 20571 67
1027275 16:19:38 CKPTINTVL 1477181:0x3392018 45.1 43.1 0.0 10 0.0
0.9 1.8 14921 346 7782 25 25211 82
1027276 16:24:36 CKPTINTVL 1477181:0xa661468 28.9 27.4 0.0 15 0.0
0.6 1.3 17436 636 8603 27 31287 99
1027277 16:29:40 CKPTINTVL 1477182:0xc4e018 4.3 3.5 0.0 2 0.0
0.4 0.5 14007 4055 7643 23 20696 63
1027278 16:36:22 CKPTINTVL 1477182:0x7f72018 103.0 102.6 0.0 0 0.0
0.0 0.0 17725 172 9144 30 36297 119
1027279 16:39:58 CKPTINTVL 1477182:0xd44a018 5.2 4.4 0.0 2 0.0
0.3 0.5 17563 4028 8092 25 22107 70
1027280 16:45:01 CKPTINTVL 1477183:0xa39b018 3.6 3.1 0.0 3 0.0
0.2 0.3 25191 8089 11422 37 47722 156
1027281 16:50:17 CKPTINTVL 1477184:0x69d0cc 16.9 14.4 0.0 13 0.1
1.1 2.2 13874 964 7435 24 21666 71
1027282 16:55:48 CKPTINTVL 1477184:0x748b018 31.1 30.0 0.0 13 0.0
0.7 1.0 21411 714 8747 27 29988 95
1027283 17:02:04 CKPTINTVL 1477185:0x9f278 96.8 95.4 0.0 8 0.0
0.5 1.0 17254 180 9177 29 30386 97
1027284 17:06:58 CKPTINTVL 1477185:0x6016018 74.9 72.8 0.0 14 0.0
1.0 1.5 16851 231 9194 29 29269 92
1027285 17:11:24 CKPTINTVL 1477185:0xe90f524 26.9 25.0 0.0 12 0.0
0.8 1.4 20564 822 10391 33 40336 128
1027286 17:18:23 CKPTINTVL 1477186:0x682f138 116.4 85.4 0.0 326 28.6
15.1 29.8 16459 192 8342 23 30949 86
1027287 17:22:29 CKPTINTVL 1477186:0x8f6e2a8 20.9 17.1 0.0 348 2.0
3.9 5.4 10849 635 5035 16 12260 39
1027288 17:27:52 CKPTINTVL 1477187:0x31d8184 19.4 17.6 0.0 24 0.2
0.9 1.8 27955 1587 13490 41 37744 117
1027289 17:33:12 CKPTINTVL 1477188:0x7f84018 12.8 12.4 0.0 16 0.2
0.4 0.4 27856 2252 19806 60 82764 253
1027290 17:39:07 CKPTINTVL 1477189:0x47801ec 55.3 16.5 0.0 247 35.1
22.2 37.4 23435 1417 12129 34 49628 142

The good news first: none of these checkpoints is performed as a blocking
one.
Nonetheless there seem to be circumstances with some of them leading to
other sorts of blockage ...

The two problems:
- occasionally slow disk (write) I/O -> look at first 'Dskflu / Sec'
column for how highly variable the flush rates are.
- occasionally long 'Ckpt Time' -> this is what would be reported as
'Avg. Txn Block Time' in online.log (a slight misnomer, but this is what we
have)

The first problem alone might not hurt any user sessions (from checkpoint's
perspective), it only would prolong the overall checkpoint duration which
doesn't hurt with non-blocking checkpoints. Of course any session activity
depending on disk (read) I/O - which usually is similarly affected - would
suffer independently from checkpoints.
The second problem could have direct blocking impact on user sessions, for
and beyond the time the checkpoint has to wait ('Ckpt Time'), longest
measured session wait time being 'Long Time', average being 'Wait Time'.

All this waiting now leading to 'critical section' which, in plain words,
is an operation that needs to complete between checkpoints, so the
instance's consistent on-disk image created by a checkpoint is guaranteed
to either contain the operation's full result or none of it.
Consequentially a 'critical section' cannot start while (a specific part
of) a checkpoint is underway, and once at least one 'critical section'
operation is underway, a new checkpoint had to wait for them to finish, no
other operations are allowed to enter a critical section during such
waiting. This waiting by a checkpoint on ongoing 'critical section'
operations is 'Ckpt Time'.

Of course a critical section normally is a smallish thing, taking
micro-seconds rather than many seconds, but there can be conditions for
longer ones too.
Now if such 'critical section' activity had to perform a lot of disk I/O
(usually for reading many pages), you'd see how the two problems could add
to each other.

One had to catch such long lasting critical section, or a checkpoint having
to wait on one, then collect some onstat outputs to learn more. In many
cases ensuring decent disk I/O performance already is sufficient for
getting things back to satisfactory performance. And of course eliminating
unnecessary disk I/O ... e.g. by increasing bufferpool sizes, lowering
LRU_MIN/MAX_DIRTY, ...

HTH,
Andreas

From: "NATYURAL HORACIO" <horacio.natyural@gmail.com>
To: ids@iiug.org
Date: 11/07/2017 05:32 PM
Subject: Re: RE: RE: RE: Informix 12.1 Blocking Checkpoints [40161]
Sent by: ids-bounces@iiug.org

I'll provide the things you need tomorrow since I don't have access right
now.

Yes, they do not contain the most buffers. THe IO rate is quite low but
also
for the other part.
Is this indicative that my IO is no longer enough ?

Do you think I have enough physical and logical logs ? Is it safe to say
that
from this, I don't have an

Based on my reading, my threadsd took a long time before it was able to
enter
the critical section.
What does this mean ? What are my threads waiting for ? If I have no block
time, then what is the wait time waiting for ?

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