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 <[EMAIL PROTECTED]> 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 > =:-> > > > ---------------------------(end of broadcast)--------------------------- TIP 3: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly