Re: Distributed transactions and asynchronous commit

Il 17/07/2013 12:52, Xenofon Papadopoulos ha scritto:
> 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
>

shared_buffers could be set up to 20-30% of the available RAM: in your
case, 16GB could be a reasonable value.

> 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

This means that auto vacuum will be triggered after around 50 updates
aech time, if your database is doing a lot of updates/inserts (as I
understood) an unnecessary amount of vacuum statements can be reached,
which will generate a lot of IO. If the inserts/updates are small, this
value could be decreased.

Giuseppe.

>
> 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
> <mailto: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 Support
> giuseppe(dot)broccolo(at)2ndQuadrant(dot)it <mailto:giuseppe(dot)broccolo(at)2ndQuadrant(dot)it> |www.2ndQuadrant.it <http://www.2ndQuadrant.it>
>
>

--
Giuseppe Broccolo - 2ndQuadrant Italy
PostgreSQL Training, Services and Support
giuseppe(dot)broccolo(at)2ndQuadrant(dot)it | www.2ndQuadrant.it