OS mirroring built into Netware 4.1 ***TEN YEARS AGO*** allowed that. I'd be more disappointed than surprised if Linux Software RAID didn't also allow that.
Parity calculation along with slicing the data into a piece for each drive will ALWAYS slow down a RAID 5 write, so it will never be as fast as a straight write to a disk, which is what a RAID 1 write is, it just happens twice simultaneously. Hardware or software, RAID 5 requires an extra step that RAID 1 doesn't whenever it writes. The necessity to seek across multiple disks will slow reads on a RAID 5 array in a similar way to reading on a fragmented disk does. (The analogy isn't perfect, but I'll go with it.) Once the data is read from the various disks, it must be stitched together into a coherent string of data. Reading on a Mirror set means that AT WORST, reads are all handled by a single operation, and occur one after the other on a single disk. At best the drives split the requests, and response time is cut in half. Either way, the queuing and reordering steps are avoided. If I make 1 read request, you are correct, the hard drive will be the limiting factor. However, if you make 2 simultaneous requests, each hard drive can respond to one. So rather than having request #2 wait until request #1 is completed, request #1 can be handled by HDD1 and request #2 can be handled by HDD2. Using the Controllers Cache would mean that either solution wouldn't be impacted by the HDDs at all. Johnny said he's building servers, so I would expect that more than 1 simultaneous request would be a very common situation. Now lets turn up the heat, and run each solution in a situation where it's missing a drive. Raid 1 will just read from the remaining disk as if it was a single disk. Raid 5 will need to calculate parity to fake the data from the missing disk. Imagine the performance hit associated with the RAID 5 solution there... Kev. ----- Original Message ----- From: "Mark Lane" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Monday, October 28, 2002 10:13 PM Subject: Re: (clug-talk) Removing RAID 1?? > > Again, I've never worked with Linux's OS RAID, so don't take my word for it. But that IS the purpose for RAID 1 afterall. Well, Fault tolerance and performance... There is no faster drive setup than mirroring. (and no, RAID 5 doesn't even come close. (well, it does come fairly close on writes, but not on reads.)) It's just really expensive because rather than n+1, you have n*2 drives. And you run out of space for 'em pretty fast on a big array. So expect a performance hit if the server is heavily used... > > > Kevin, unless you have a very highend raid card, raid 1 is not going to > be faster than any one drive on reads. Only high-end cards allow you to > read for either drive, most don't do that. > > > > >
