Re: Table and Index compression

On Fri, 07 Aug 2009 15:42:35 +0200, Kevin Grittner
<Kevin(dot)Grittner(at)wicourts(dot)gov> wrote:

> Pierre Frédéric Caillaud<lists(at)peufeu(dot)com> wrote:
>
>> tablespace is a RAID5 of 3 drives, xlog in on a RAID1 of 2 drives,
>> but it does it too if I put the tablespace and data on the same
>> volume.
>
>> it starts out relatively fast :
>>
>> si so bi bo in cs us sy id wa
>> 0 0 0 43680 2796 19162 42 18 37 3
>> 0 0 0 45515 3165 20652 44 17 35 4
>> 0 0 0 43130 3046 21991 43 17 38 2
>>
>> then here it starts to slow down : check "bo" output
>>
>> 0 0 181 24439 577 3541 31 6 40 23
>> 0 0 176 17258 292 1324 31 4 43 22
>> 0 0 0 18626 162 693 35 3 49 12
>> 0 0 1 21554 235 1362 31 5 50 14
>> 0 0 0 19177 324 2053 35 4 50 12
>> 0 0 0 19208 206 1155 36 4 48 12
>> 0 0 1 20740 215 1117 33 4 50 13
>> 0 0 0 20154 258 1100 32 4 50 14
>> 0 0 0 20355 316 2056 34 5 49 12
>>
>> ... and it stays like this until the end of the INSERT...
> I don't know if this is it, but we tend to see outrageously high
> performance at the start of a benchmark because of the battery-backed
> cache in the RAID controller. Every write comes back immediately
> after copying the data to RAM. After a while the cache gets filled
> and you settle down to a steady state. If it's not BBU with
> write-back enabled, perhaps you have drives that lie about write
> completion?
> -Kevin
>

I'm answering my own question : at the beginning of the run, postgres
creates a 800MB temporary file, then it fills the table, then deletes the
temp file.
Is this because I use generate_series to fill the test table ?