Hello,
As blocksizes, random I/O and linear I/O are critical I/O performance
parameters I had a look on PostgreSQL and a commercial software vendor.
Therefore I enhanced the system tap script:
http://www.wiesinger.com/opensource/systemtap/disktop_gw.stp
Output per 5 seconds on a sequence scan:
UID PID PPID CMD DEVICE T BYTES
REQUESTS BYTES/REQ
26 4263 4166 postmaster dm-1 R 168542208
20574 8192
=> 32MB/s
So I saw, that even on sequential reads (and also on bitmap heap scan
acces) PostgreSQL uses only 8k blocks. I think that's a major I/O
bottleneck.
A commercial software database vendor solved the problem by reading
multiple continuous blocks by multiple 8k blocks up to a maximum
threshold. Output per 5 seconds on an equivalent "sequence scan":
UID PID PPID CMD DEVICE T BYTES
REQUESTS BYTES/REQ
1001 5381 1 process dm-1 R 277754638
2338 118800
=> 53 MB/s
A google research has shown that Gregory Stark already worked on that
issue (see references below) but as far as I saw only on bitmap heap
scans.
I think this is one of the most critical performance showstopper of
PostgreSQL on the I/O side.
What's the current status of the patch of Gregory Stark? Any timeframes
to integrate?
Does it also work for sequence scans? Any plans for a generic "multi block
read count" solution?
Any comments?
Thnx.
Ciao,
Gerhard
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http://www.wiesinger.com/
http://wiki.postgresql.org/wiki/Todo#Concurrent_Use_of_Resources
http://archives.postgresql.org/pgsql-hackers/2007-12/msg00027.php
http://archives.postgresql.org/pgsql-hackers/2007-12/msg00395.php
http://archives.postgresql.org/pgsql-hackers/2007-12/msg00088.php
http://archives.postgresql.org/pgsql-hackers/2007-12/msg00092.php
http://archives.postgresql.org/pgsql-hackers/2007-12/msg00098.php
http://archives.postgresql.org/pgsql-hackers/2006-10/msg00820.php
http://markmail.org/message/a5osy4qptxk2jgu3#query:+page:1+mid:hz7uzhwxtkbzncy2+state:results
http://markmail.org/message/a5osy4qptxk2jgu3#query:+page:1+mid:a5osy4qptxk2jgu3+state:results
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