>> Ah that makes total sense now, thanks. Do the 3wire and the Areca cards >> allow you to remove battery/cache/disk and install into similar motherboard? >> Also >> when you say remove battery and cache, do you mean remove the entire RAID >> card with battery attached to it as complete assembly with accompaying drive >> and slap them all onto a new motherboard? >> >> > > I /think/ with the 3ware you remove and swap the whole card, along with > the drives. > > On many server grade systems, such as the the HP DL380 series, with on > board SmartArray, the cache ram module and battery are separate > detachable components. in the dl380 they are actually two pieces with a > cord between them. you unclip and remove the battery from the chassis > without messing with the wire, then you pull the cache module out of its > special slot, these can then be installed in another HP smartarray, > along with the drives from the original system, and when that new DL380 > powers up, the raid controller will verify the drives, and flush its > cache, insureing data integrity, then boot up your environment. > > >>>> Again pardon my ignorance, what is a hot spare? A blank drive connected >>>> in the RAID 5 setup that can be written to in case one of the other 3 >>>> drives fail? >>>> >>>> >>>> >>> exactly. a hot spare sits unused until one of the RAID members fails, >>> then its used to replace the failed drive by remirroring or restriping >>> the parity, once this is finished, and the original failed drive is >>> replaced it can become the new hot spare. >>> >> >> So if I understand correctly, RAID 5 is three active drives and one blank >> drive connected to a RAID 5 card, >> and if one of the three active drives fails, the fourth empty drive is >> automatically written to? If correct, what happens if the drive that fails >> loses all it's data before the >> blank drive has a chance to grab it? >> > > > with a 3 drive raid 5, you write two drives worth of data across the > 3... every third 'block' is a 'parity block' calculated by bit-wise > exclusive or (XOR) of the other two blocks. on a 3 drive RAID-5, this > parity block alternates across all three drives.... > > drive: 0 1 2 > =========== > data 0 1 0x1 > blocks 2x3 2 3 > 4 4x5 5 > 6 7 6x7 > 8x9 8 9 > ..... > > each of those 'blocks' is like 32K bytes, 64 x 512 byte sectors (this is > the stride of the raid, configured when you create the raid). the ones > that are just numbers are your data blocks, while the 0x1 is (block_0 > XOR block_1) eg, the parity block for that stripe. > > if any one drive, /dies/ abruptly with no warning, you can still read > all the data from the remaining drives, the missing drive is the XOR of > the other drives, so the controller can reconstruct it on the fly and > you will continue operating in a degraded performance mode. > > if you have a spare drive, or when you replace the failed drive, the > raid controller begins a rebuild where it reads ALL the blocks of the > working drives, XOR's them together, and writes this to the spare/new > drive. when its done, things revert to normal full performance and > redundant operation. raid controllers can do this while the logical > volume is still in use and online, many let you set the priority of this > to lower the performance impact from raid rebuilds > > you can extend this with a reasonable number of drives, for instance, 5 > drives might look like... > > > drive: 0 1 2 3 4 > ===================== > data 0 1 2 3 P > blocks P 4 5 6 7 > 8 P 9 10 11 > .... > > where the P's are XOR's of /all/ the other blocks on the same line. p0 > = b0 X b1 X b2 X b3. p1 = b4 X b5 X b6 X b7, etc. > > there's tons of material online explaining this stuff far better than a > centos list can. > http://en.wikipedia.org/wiki/RAID
Ah great i'll check out the URL thanks. One thing, an earlier poster reccomended RAID 5 instead of RAID 1. I guess if one only has 2 drives RAID 1 is the way to go but if I have 4 drives he said go with RAID 5 over RAID 1. Isn't RAID 1 mirroring a better solution for a 4 drive array or am I missing something here? _________________________________________________________________ Need to know the score, the latest news, or you need your Hotmail®-get your "fix". http://www.msnmobilefix.com/Default.aspx_______________________________________________ CentOS mailing list CentOS@centos.org http://lists.centos.org/mailman/listinfo/centos
