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.brocc...@2ndquadrant.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.brocc...@2ndquadrant.it | > www.2ndQuadrant.it > >
