On 10/4/2011, at 2:50pm, Mark Knecht wrote: > ... loses 1 drive and > then, while in the process of fixing the RAID, loses a second drive. > Most of us (myself included) buy identical drives all at the same time > from the same vendor. This means all the drives were likely from the > same manufacturing batch and, if they are drives that will fail at all > then the group will likely experience multiple drive failures.
It doesn't make it *likely* that they'll fail simultaneously. It makes it less unlikely. > The > underlying idea of RAID is that the drives are not likely to fail at > the same time giving us time to fix the array. However, if /dev/sda > fails the chances of /dev/sdb failing is higher if they were built at > the same time in the same plant. ^ This is a more accurate synopsis. > Reading the mdadm list for the last couple of years it seems that many > folks running data centers intentionally buy drives from multiple > manufactures, or drives of different sizes from the same manufacturer, > hoping to lower the chances of multiple failures at the same time. I've found it sometimes quite inconvenient to do this, and whilst I consider it good practice I get the impression a lot of people, perhaps the majority, don't bother (or don't even know they should). I kinda think it's a nice thing to do but not essential - I don't know that the risk of simultaneous failure is increased that significantly. Many high-end servers will be sold off-the-shelf by their manufacturers with consecutively-serialed drives in the RAID array - I don't think this is considered risky enough for Dell or IBM to offer non-matching drives as a purchasing option. One also has to wonder what the performance implications might be of having three drives in an array with slightly different rotational speeds, spin-up and seek times. Ultimately, we shouldn't be fully dependent upon RAID for the integrity of our data, anyway. "RAID is not a backup" is the famous saying. > As for hardware RAID the risk I hear about there is that if the > controller itself fails then you need an identical backup controller > or you risk the possibility that you won't be able to recover > anything. I don't know how true that is or whether it's just FUD. Generally you just need a similar one. In the case of 3ware you can connect your drives to any other 3ware controller and it will recognise the array descriptors written at the start of the drive. I haven't swapped drives between the PERCs (rebadged Adaptec, I think) of Dell 2650s & 2850s, but these machines are now so cheap on the secondhand market anyway, you can afford to have a spare identical one. I think you're over-estimating the *risk* of being unable to find a RAID controller of the same model. But certainly if you buy a good PCIe SATA card on the secondhand market it will not be cheap to replace in the event of failure, and a bargain may not come up on eBay immediately. So I think you'll certainly be able to recover your data, you may just some inconvenience of having to wait to find a cheap enough card or spend a lot of money buying an obsolete card in a hurry. Ideally, you have a spare in advance or buy hardware RAID under a 5-year warranty (in which case it's replaced next-day by the manufacturer). This is really a matter of horses-for-courses. Most people (including myself) don't really "need" hardware RAID. Hardware RAID is much more expensive, but I do consider it "better", if only because you can hot-swap. That is not assured with cheap SATA controllers. OTOH Linux's software RAID does seem to be just as fast (??) as hardware RAID, and has some cool features. Stroller.
