Date: Thu, 7 Jun 2001 13:55:52 -0700 From: Michael VanLoon <MichaelV@EDIFECS.COM> To: "'Achim Patzner'" <ap@bnc.net> Cc: hardware@freebsd.org Subject: RE: Casing wanted Message-ID: <36F7B20351634E4FBFFE6C6A216B30D54C24@ecx1.edifecs.com>
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Proof of concept is fine. I wish you luck on building this server and its long-term reliability. So with that, I'm not trying to convince you not to build it, but I figure I should address the points I already brought up. > From: Achim Patzner [mailto:ap@bnc.net] > Sent: Thursday, June 07, 2001 1:24 AM > > > Agreed it's quite expensive but... with hardware SCSI RAID you get: > > - You don't have all these squirrelly issues you just > brought up (cable > lengths, etc.) > > Which is about the only problem I encountered and 3ware just > told me (after > a direct hit on their long term memory with a hard object) > that there are > cables > up to 70 cm. Yes but as far as I know these are outside the ATA100 spec, and are not guaranteed to work in all cases. It's kinda hit-and-miss. If it works, you're good (unless it starts acting strange). If it doesn't work, oh well. Correct me if I'm wrong on this point -- I don't have an ATA100 spec in front of me. > > - Automatic unattended hardware failover to hot-spare(s) > > - Automatic unattended background fill-in of data on > failed-in hot spares > > while server is live > > - Caching controller that does delayed elevator-sorted > write-backs, and > > read-ahead > > This is nothing the 3ware controller won't do. Well that hasn't been proven to me. 3ware's specs and white-papers are unfortunately alarmingly light on the actual details of how complete their live hot-spare support actually is. Although this doesn't confirm that they don't do it as well, it's at least noting that they don't spend any time saying more than a bullet point "hot-spare". A good hardware SCSI RAID controller will fill in the hot-spare while the server is up and live. Without the server even realizing anything happened (aside from reduced disc performance) it will back-fill the redundant data onto the new drive and bring it into the array. Once the new drive is filled, it is a full member of the array and the machine is running exactly as it was before the failure. All this without a reboot, and without any downtime. One other thing I forgot to mention is dynamic expansion of arrays. If you need to add more drives, it will use the same technology to expand the size of the array without having to actually move any data (it will redistribute the data on the disks for optimal striping, but that is invisible to the user or OS). Then if you're running NT you just tell it to expand the Volume, and if you're running FreeBSD you just run growfs. Or make a new partition there if that suits you. Without detailed white-papers it's hard to claim the 3ware controller can do all this. Maybe it can but I don't see any proof of it in their documentation. > > - Better reliability (yes SCSI drives really ARE built better) > > No. I had 15 IBM DDYS (of 35) failing after less than 12 > months and didn't > lose > a single Maxtor. Everybody is going to have wildly varying accounts of this over such a small sample size. I had 6 of 8 Western Digital IDE drives fail within 3 days of buying them. But I realize this is just an anomaly and we got a bad batch. Over the span of several years and hundreds or thousands of drives, I think you will find the results generally go in SCSI's favor. > > - Higher performance (though yes, IDE performance is pretty good) > > Hm. I've seen people getting 95 MB/s through a 3ware RAID. > Don't forget that > it got a single channel per disk. > > All in all this is a reason why they want this machine - they want to > compare > performance... Fine, I'm all for comparing performance. Competition is good! However keep in mind that modern SCSI controllers are 160MB/s per channel. With four channels that's 740MB/s. And that's just standard Ultra3 LVD, not fiber-channel. > > - Depending on the controller from 15 to 60 drives per controller > > Not really. I'm a strong believer in one channel per disk. Well you're looking at it from an IDE point of view. IDE drives REQUIRE one drive per controller to get decent performance, because IDE is a much simpler protocol. And four channels gives you fine fault tolerance, especially if you're using a technology like RAID-10 (also called 0+1), where you spread your mirror drives over multiple busses. With tagged-command-queuing, disconnect/reconnect, etc. SCSI drives can share a bus without stealing all the bandwidth. A modern drive can't do any more than around 40MB/s sustained in optimal conditions anyway (and that drops significantly if any seeking is involved). 4 drives per channel = 160. 4 channels X 4 drives = 16 drives = 740MB/s. That adds up to higher performance on my calculator. Of course these numbers are totally theoretical, and you will get nowhere near that performance in the real world, on a production filesystem, whether it be on SCSI or IDE. > > - Higher quality cases, hot-swap cartridges, etc. on the market > > Definitely not. The best hot swap cartridge I've ever seen was > an IDE cartridge. I thought someone mixed Dark Vader and the Cylons > and turned them into a status display Status displays don't equal high-quality hot-swap equipment. > > So it's not like you're paying more for nothing. There are > some very > > substantial benefits, especially in the reliability/uptime > > department when a disk fails -- no need to bring the server down, or > > even be there when it swaps in a hot spare and starts using it. > > Nothing I wouldn't get wit IDE too... Once again the documentation is too light on all the details to confirm this. It may be true but their documentation doesn't give me enough details to confirm it. Just a counter-point. :-) I'm not trying to dissuade you from building this and verifying it works. However if you want a true and fair comparison, you need to open-mindedly compare it with all that SCSI RAID has to offer and judge the plusses and minuses of each platform. If you're really interested in comparing, I'd suggest starting your SCSI research here: http://www.adaptec.com/worldwide/product/prodfulldesc.html?prodkey=ASR-3400S &cat=%2fTechnology%2fRAID%2fRAID+for+Mid-Range+Servers There are lots of other good SCSI RAID also, this is just one of several. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hardware" in the body of the message
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