Not true - the recommended RAID level is RAID 10, not RAID 0+1 (at
least I would never recommend 1+0 for anything).
RAID 10 and RAID 0+1 are _quite_ different. One gives you very good
redundancy, the other is only slightly better than RAID 5, but
operates faster in degraded mode (single drive).
Alex Turner
netEconomist
On 4/18/05, John A Meinel <john@arbash-meinel.com> wrote:
> Alex Turner wrote:
>
> >[snip]
> >
> >
> >>Adding drives will not let you get lower response times than the average seek
> >>time on your drives*. But it will let you reach that response time more often.
> >>
> >>
> >>
> >[snip]
> >
> >I believe your assertion is fundamentaly flawed. Adding more drives
> >will not let you reach that response time more often. All drives are
> >required to fill every request in all RAID levels (except possibly
> >0+1, but that isn't used for enterprise applicaitons).
> >
> Actually 0+1 is the recommended configuration for postgres databases
> (both for xlog and for the bulk data), because the write speed of RAID5
> is quite poor.
> Hence you base assumption is not correct, and adding drives *does* help.
>
> >Most requests
> >in OLTP require most of the request time to seek, not to read. Only
> >in single large block data transfers will you get any benefit from
> >adding more drives, which is atypical in most database applications.
> >For most database applications, the only way to increase
> >transactions/sec is to decrease request service time, which is
> >generaly achieved with better seek times or a better controller card,
> >or possibly spreading your database accross multiple tablespaces on
> >seperate paritions.
> >
> >
> This is probably true. However, if you are doing lots of concurrent
> connections, and things are properly spread across multiple spindles
> (using RAID0+1, or possibly tablespaces across multiple raids).
> Then each seek occurs on a separate drive, which allows them to occur at
> the same time, rather than sequentially. Having 2 processes competing
> for seeking on the same drive is going to be worse than having them on
> separate drives.
> John
> =:->
>
>
>
