Date: Tue, 17 Feb 98 16:45:54 -0500 From: curt@kcwc.com (Curt Welch) To: freebsd-scsi@FreeBSD.ORG Subject: Re: getting oriented with RAID Message-ID: <9802172145.AA00337@mail.kcwc.com>
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> I've been reading in the archives and on Simon's website > http://simon-shapiro.org), but there are some basic things > I'm still not clear on that I'm hoping y'all could > explain. I'll comment on what I know about. > 1) From the DPT website, their PM2044UR (single channel > PCI) RAID controller supports RAID 0, 1, 5, 0+1, and 0+5. > Does this necessarily mean that the FreeBSD driver written > by Simon can also do all of the above? Something I read > in the archives indicated that you need do use ccd for > RAID0. You can do all of those (at least on the PM3334), but as you read, you must use the FreeBSD ccd driver to do the RAID 0 (striping) parts. DPT supports RAID 0 across multiple controllers, which means it must be supported in the driver and not on/in the controller. As far as I can tell, Simon just didn't get around to adding that to the driver. I see no real reason to do so since it's supported by ccd with no problem. ccd is very easy to use and you would have to use it if you wanted to stripe across DPT and non-DPT controllers anyway. 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). > 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. 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
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