Re: Distributed transactions and asynchronous commit

Thank you for your replies so far.
The DB in question is Postgres+ 9.2 running inside a VM with the following
specs:

16 CPUs (dedicated to the VM)
60G RAM
RAID-10 storage on a SAN for pgdata and pgarchieves, using different LUNs
for each.

We have 3 kind of queries:

- The vast majority of the queries are small SELECT/INSERT/UPDATEs which
are part of distributed transactions
- A few small ones, which are mostly SELECTs
- A few bulk loads, where we add 100k - 1M of rows in tables

Our settings are:

shared_buffers: 8G
work_mem: 12M
checkpoint_segments: 64

Autovacuum is somewhat aggressive, as our data changes quite often and
without it the planner was completely off.
Right now we use:

autovacuum_analyze_scale_factor: 0.1
autovacuum_analyze_threshold: 50
autovacuum_freeze_max_age: 200000000
autovacuum_max_workers: 12
autovacuum_naptime: 10s
autovacuum_vacuum_cost_delay: 20ms
autovacuum_vacuum_cost_limit: -1
autovacuum_vacuum_scale_factor: 0.2
autovacuum_vacuum_threshold: 50

At high-peak hour, the disk utilization for the pgdata mountpoint is:

*00:00:01 DEV tps rd_sec/s wr_sec/s avgrq-sz avgqu-sz
await svctm %util*
13:20:01 dev253-2 7711.62 24166.97 56657.95 10.48 735.28
95.09 0.11 86.11
13:30:01 dev253-2 5340.88 19465.30 39133.32 10.97 319.20
59.94 0.15 82.30
13:40:01 dev253-2 2791.02 13061.76 19330.40 11.61 349.95
125.38 0.33 90.73
13:50:01 dev253-2 3478.69 10503.84 25505.27 10.35 308.12
88.57 0.20 68.12
14:00:01 dev253-2 5269.12 33613.43 35830.13 13.18 232.48
44.09 0.19 100.05
14:10:01 dev253-2 4910.24 21767.22 33970.96 11.35 322.52
65.64 0.21 104.55
14:20:02 dev253-2 5358.95 40772.03 33682.46 13.89 721.81
134.32 0.20 104.92
14:30:01 dev253-2 4420.51 17256.16 33315.27 11.44 336.53
76.13 0.15 65.25
14:40:02 dev253-2 4884.13 28439.26 31604.76 12.29 265.32
54.26 0.20 97.51
14:50:01 dev253-2 3124.91 8077.46 22511.59 9.79 50.41
16.13 0.24 76.17

and for pgarchives:

*00:00:01 DEV tps rd_sec/s wr_sec/s avgrq-sz avgqu-sz
await svctm %util*
13:20:01 dev253-3 2802.25 0.69 22417.32 8.00 465.05
165.94 0.02 4.32
13:30:01 dev253-3 1559.87 11159.45 12120.99 14.92 64.17
41.11 0.08 12.02
13:40:01 dev253-3 922.62 8066.62 7129.15 16.47 19.75
21.40 0.08 6.99
13:50:01 dev253-3 1194.81 895.34 9524.53 8.72 28.40
23.76 0.01 1.69
14:00:01 dev253-3 1919.12 0.46 15352.49 8.00 51.75
26.95 0.01 1.61
14:10:01 dev253-3 1770.59 9286.61 13873.79 13.08 139.86
78.97 0.08 14.46
14:20:02 dev253-3 1595.04 11810.63 12389.08 15.17 109.17
68.42 0.15 24.71
14:30:01 dev253-3 1793.71 12173.88 13957.79 14.57 141.56
78.89 0.08 13.61
14:40:02 dev253-3 1751.62 0.43 14012.53 8.00 43.38
24.76 0.01 1.40
14:50:01 dev253-3 1351.72 3225.19 10707.29 10.31 31.91
23.59 0.02 2.93

On Wed, Jul 17, 2013 at 1:09 PM, Giuseppe Broccolo <
giuseppe(dot)broccolo(at)2ndquadrant(dot)it> wrote:

> Hi,
>
> Il 17/07/2013 09:18, Xenofon Papadopoulos ha scritto:
>
> In the asynchronous commit documentation, it says:
>
> *The commands supporting two-phase commit, such as PREPARE TRANSACTION,
> are also always synchronous
> *
>
> Does this mean that all queries that are part of a distributed
> transaction are synchronous?
>
> In our databases we have extremely high disk I/O, I'm wondering if
> distributed transactions may be the reason behind it.
>
>
> Distributed transactions are base on two-phase-commit (2PC) algorithms for
> ensuring correct transaction completion, so are synchronous.
> However, I think this is not the main reason behind your extremely high
> disk I/O. You should check if your system is properly tuned to get the best
> performances.
> First of all, you could take a look on your PostgreSQL configurations, and
> check if shared_memory is set properly taking into account your RAM
> availability. The conservative PostgreSQL default value is 24 MB, forcing
> system to exploit many disk I/O resources.
> Aside from this, you could take a look if autovacuum is often triggered
> (generating a large amount of I/O) in case of large use of updates/inserts
> in your database.
>
> Regards,
>
> Giuseppe.
>
> --
> Giuseppe Broccolo - 2ndQuadrant Italy
> PostgreSQL Training, Services and Supportgiuseppe(dot)broccolo(at)2ndQuadrant(dot)it | www.2ndQuadrant.it
>
>