Date: Wed, 18 Feb 1998 00:10:54 -0800 (PST) From: Simon Shapiro <shimon@simon-shapiro.org> To: (Curt Welch) <curt@kcwc.com> Cc: freebsd-scsi@FreeBSD.ORG Subject: Re: getting oriented with RAID Message-ID: <XFMail.980218001054.shimon@simon-shapiro.org> In-Reply-To: <9802172145.AA00337@mail.kcwc.com>
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On 17-Feb-98 Curt Welch wrote: ... > If you define a RAID 0 array with the DPT manager, it just > shows up as separate drives when you boot instead of as one > large drive (like it does with a RAID 1 or RAID 5 array). Use dptmgr/fw0, or tell dptmgr, during initial installation that your O/S is Linux, or both. I forget which. It will then behave properly. I have several such arrays on nomisd-sendero (the arrays are shared between the two machines). >> 3) Given that you want redundancy and optimal performance, >> can you increase performance by running RAID0+1, or is >> it better to just run RAID1? > > I guess you need to explain a little more about what you > are asking. > > I'd say that four 4.5gig drives in a RAID 0+1 config will > perform better than two 9gig drives in a RAID 1 config. > Both of these configs give you the same overall disk space > (9gig), but with the smaller drives striped together, you > spread the IO over more spindles (4 vs 2) and therefor you > will get better performance. So, in this case 0+1 is better > than just 1. For most random access applications, the more heads the marrier. One of the biggest secrets in RDBMS benchmarking is disk optimization. What you see, almost universally, is that, for high-end systems, the benchmarking engineer rarely uses more than 300MB per drive, and rarely puts more than 4-6 drives on one SCSI bus. Before you jump, Unix ffs filesystems, make, gcc, etc. are a different bird. They typically read large chunks sequentially. OLTP transactions are very small and cache hit ratios deteriorate with database size - fast. Unless you want to do what I used to do for a living for many years (compute all this nonsense), just experiment with drives, busses, stripe sizes, amount of cache, cache utilization, host cache vs. DPT cache, etc. > But if you have 4 drives which you can leave as two RAID-1 > arrays or combine them into one large RAID 0+1 array, then > performance won't change much. > > Curt Welch > > To Unsubscribe: send mail to majordomo@FreeBSD.org > with "unsubscribe freebsd-scsi" in the body of the message ---------- Sincerely Yours, Simon Shapiro Shimon@Simon-Shapiro.ORG Voice: 503.799.2313 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-scsi" in the body of the message
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