| Luke,
Interesting numbers. I'm a little concerned about the use of blockdev —setra 16384. If I understand this correctly it assumes that the table is contiguous on the disk does it not ?
Dave
On 18-Nov-05, at 10:13 AM, Luke Lonergan wrote: Dave,
On 11/18/05 5:00 AM, "Dave Cramer" <pg@xxxxxxxxxxxxx> wrote:
>
> Now there's an interesting line drawn in the sand. I presume you have
> numbers to back this up ?
>
> This should draw some interesting posts.
Part 2: The answer
System A:
This system is running RedHat 3 Update 4, with a Fedora 2.6.10 Linux kernel.
On a single table with 15 columns (the Bizgres IVP) at a size double memory (2.12GB), Postgres 8.0.3 with Bizgres enhancements takes 32 seconds to scan the table: that’s 66 MB/s. Not the efficiency I’d hope from the onboard SATA controller that I’d like, I would have expected to get 85% of the 100MB/s raw read performance.
So that’s $1,200 / 66 MB/s (without adjusting for 2003 price versus now) = 18.2 $/MB/s
Raw data:
[llonergan@kite4 IVP]$ cat scan.sh
#!/bin/bash
time psql -c "select count(*) from ivp.bigtable1" dgtestdb
[llonergan@kite4 IVP]$ cat sysout1
count
----------
10000000
(1 row)
real 0m32.565s
user 0m0.002s
sys 0m0.003s
Size of the table data:
[llonergan@kite4 IVP]$ du -sk dgtestdb/base
2121648 dgtestdb/base
System B:
This system is running an XFS filesystem, and has been tuned to use very large (16MB) readahead. It’s running the Centos 4.1 distro, which uses a Linux 2.6.9 kernel.
Same test as above, but with 17GB of data takes 69.7 seconds to scan (!) That’s 244.2MB/s, which is obviously double my earlier point of 110-120MB/s. This system is running with a 16MB Linux readahead setting, let’s try it with the default (I think) setting of 256KB – AHA! Now we get 171.4 seconds or 99.3MB/s.
So, using the tuned setting of “blockdev —setra 16384” we get $6,000 / 244MB/s = 24.6 $/MB/s
If we use the default Linux setting it’s 2.5x worse.
Raw data:
[llonergan@modena2 IVP]$ cat scan.sh
#!/bin/bash
time psql -c "select count(*) from ivp.bigtable1" dgtestdb
[llonergan@modena2 IVP]$ cat sysout3
count
----------
80000000
(1 row)
real 1m9.875s
user 0m0.000s
sys 0m0.004s
[llonergan@modena2 IVP]$ !du
du -sk dgtestdb/base
17021260 dgtestdb/base
Summary:
<cough, cough> OK – you can get more I/O bandwidth out of the current I/O path for sequential scan if you tune the filesystem for large readahead. This is a cheap alternative to overhauling the executor to use asynch I/O.
Still, there is a CPU limit here – this is not I/O bound, it is CPU limited as evidenced by the sensitivity to readahead settings. If the filesystem could do 1GB/s, you wouldn’t go any faster than 244MB/s.
- Luke
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