Date: Wed, 18 Feb 1998 00:02:30 -0800 (PST) From: Simon Shapiro <shimon@simon-shapiro.org> To: Omar Thameen <omar@clifford.inch.com> Cc: freebsd-scsi@FreeBSD.ORG Subject: RE: getting oriented with RAID Message-ID: <XFMail.980218000230.shimon@simon-shapiro.org> In-Reply-To: <19980217150134.49019@clifford.inch.com>
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On 17-Feb-98 Omar Thameen wrote: > Hi, > > With DPT's single channel RAID controllers being priced at under $800 > retail, we're looking into adding it to several of our systems. > > 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. > > 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. Let me clarify this a bit; DPT always creates RAID-5 arrays by using the firmware on the HBA, and these arrays are confined to one controller (i.e. no more than 7-45 drives, depending on the controller and options). RAID-0 (striping) and RAID-1 (mirroring can be created as either firmware controlled arrays (run dptmggr with /fw0 option), or as in-O/S driver code. Now, with the in-kernel RAID-{0,1} you can even create RAID-0 arrays that are composed of other, redundant arrays. An example says it the best: We have a PM3334UW, with 3 SCSI busses. Each bus has 4 disks, for a total of 12 drives. The drives are all 1GB each. All the drives are setup to be target ID 0-3, This means that our system has drives 0-0, 0-1, 0-2, 0-3, 1-0, 1-1, 1-2, 1-3, 2-0, 2-1, 2-2, 2-3. Using dptmgr, we created 2 RAID-5 arrays, 3 disks in each, 1 RAID-1 array, and 1 disk designated Hot Spare: Array 0: 0-0, 1-0, 2-0 = 2GB usable/visible space Array 1: 0-1, 1-1, 2-1 = 2GB usable/visible space Array 2: 0-2, 1-2, 2-2 = 2GB usable/visible space Array 3: 0-3, 1-3 = 1GB usable/visible space Hot Spare: 2-3 Invisible to the system Now, with my driver, on FreeBSD, this is where it ends for the present time. If, in this example, you cannot live with 3x2GB drives, you have to stripe arrays 0,1,2 together using ccd, and have one ``drive'' with a capacity of 6GB. This array will be very fault tolerant and rather fast (compared to RAID-5 arrays. Actually, you can concatenate Array 3 to the other,s but let's keep it simple. With DOS, Win95, UnixWare, etc. You cna have the DPT create the 6GB super-array. this is called 0+5, or some such. Why can't we do it in FreeBSD? I have not done it yet; To provide this facility, I have to merge a lot of code that will NOT pass ANY FreeBSD coding style. The code literally have to be re-typed. This takes time, and since ccd does the same thing, it has a low priority. > 2) Do the FreeBSD drivers support the PM2044UR card, or do you have > to use the higher-end, multi-channel product? Yes. No. Any PCI card is already in -current. The EISA patches will go in for review later this week. Unlike some other SCSI HBA manufacturers, ALL DPT cards speak the same exact language. So, once the probing and attaching of the driver to the hardware is done, the rest of the driver is unchanged. to be totally honest, some older cards have some minor firmware bugs that has to be worked around. Really old ISA cards, do not have DMA support. These will not work with the driver as it stands today. the change is nimor, on the order of 20-50 lines of code, but it is really a waste of time, as they are out of production for many years. > 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? READ performance of RAID-1 is similar to TAID-0 across two drives. WRITE perfromance is similar to a single drive. READ performance of RAID-5 is similar to RAID-0 of the same number of stripes. WRITE perfromance is much less. On the order of 5-6MB/Sec. The next generation controllers do RAID-5 parity in hardware and have many more MIPS in the processor, so RAID-5 perfromance is greatly enhanced. > > Thanks, > Omar > > 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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