Re: Distributed transactions and asynchronous commit

wal_buffers: 32M
effective_io_concurrency: 4

There is no bloat.
Note that we are using Postgres inside a VM, there is a VMFS layer on top
of the LUNs which might affect the performance. That said, we're still
wondering if this much I/O is normal and if we can somehow reduce it.
Enabling async commits in a DB without distributed transactions resulted to
a huge decrease in I/O, here there was almost no effect.

On Wed, Jul 17, 2013 at 2:21 PM, Vasilis Ventirozos
<v(dot)ventirozos(at)gmail(dot)com>wrote:

>
>
>
> On Wed, Jul 17, 2013 at 11:52 AM, Xenofon Papadopoulos <xpapad(at)gmail(dot)com>wrote:
>
>> 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
>>
>
> settings look ok, except vacuum and analyze threshold that is in my
> opinion too agressive (500 would make more sense) and workers at 6 you
> haven't mentioned wal_buffers and effective_io_concurrency settings but i
> dont think that it would make much of a difference
>
>>
>>
>> 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
>>
>
> assuming that sector = 512 bytes, it means that your san makes 20mb/sec
> read which if its not totally random-reads is quite low,
> i would start from there, make tests to see if everything works ok,
> (bonnie++, dd , etc) and if you are getting the numbers you are supposed to
>
>
>> 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
>>>
>>>
>>
>