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iostat question

From: jelle <jellej(at)pacbell(dot)net>
To: Postgresql Performance <pgsql-performance(at)postgresql(dot)org>
Subject: iostat question
Date: 2004-10-21 00:28:08
Message-ID: Pine.LNX.4.44.0410201609590.6618-100000@localhost.localdomain (view raw or flat)
Thread:
Lists: pgsql-performance
Hello All,

I have an iostat question in that one of the raid arrays seems to act 
differently than the other 3. Is this reasonable behavior for the 
database or should I suspect a hardware or configuration problem? 

But first some background: 
Postgresql 7.4.2 
Linux 2.4.20, 2GB RAM, 1-Xeon 2.4ghz with HT turned off
3Ware SATA RAID controller with 8 identical drives configured as 4 
      RAID-1 spindles
64MB RAM disk

postgresql.conf differences to postgresql.conf.sample:
tcpip_socket = true
max_connections = 128
shared_buffers = 2048
vacuum_mem = 16384
max_fsm_pages = 50000
wal_buffers = 128
checkpoint_segments = 64
effective_cache_size = 196000
random_page_cost = 1
default_statistics_target = 100
stats_command_string = true
stats_block_level = true
stats_row_level = true

The database is spread over 5 spindles:
/ram0 holds the busiest insert/update/delete table and assoc. indexes for
      temporary session data
/sda5 holds the OS and most of the tables and indexes
/sdb2 holds the WAL
/sdc1 holds the 2nd busiest i/u/d table (70% of the writes)
/sdd1 holds the single index for that busy table on/sdc1

Lately we have 45 connections open from a python/psycopg connection pool.
99% of the reads are cached.
No swapping.

And finally iostat reports:

Device:    rrqm/s wrqm/s   r/s   w/s  rsec/s  wsec/s    rkB/s    wkB/s 
avgrq-sz avgqu-sz   await  svctm  %util
/dev/sda5    0.01   3.32  0.01  0.68    0.16   32.96     0.08    16.48    
48.61     0.09   12.16   2.01   0.14
/dev/sdb2    0.00   6.38  0.00  3.54    0.01   79.36     0.00    39.68    
22.39     0.12    3.52   1.02   0.36
/dev/sdc1    0.03   0.13  0.00  0.08    0.27    1.69     0.13     0.84    
24.06     0.13  163.28  13.75   0.11
/dev/sdd1    0.01   8.67  0.00  0.77    0.06   82.35     0.03    41.18   
107.54     0.09   10.51   2.76   0.21

The /sdc1's await seems awfully long compared to the rest to the stats.
                                                                                
Jelle


-- 

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Osama-in-October office pool.


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