On Sat, Nov 08, 2008 at 03:45:38AM -0600, Mark Allums wrote: > lee wrote: > >On Fri, 7 Nov 2008 23:29:55 -0500 > >"Douglas A. Tutty" <[EMAIL PROTECTED]> wrote: > > > >>On Fri, Nov 07, 2008 at 10:20:51PM -0600, Mark Allums wrote: > >> > >>If you do only have three drives, add it to the raid1 array. > > > >Hm, if you do that, is there any other use for the third disk than as a > >spare? > > > I think my name did not add appreciably to the subject, here, but, to > answer the question, I didn't understand what Douglas meant. In a RAID > 0, (striping) you can stripe across any number of drives. In RAID 1 > (mirroring) you can mirror one drive any number of times. I am not sure > what the point would be of the latter. > > Now, often, one can "migrate" an array from one type to another, e.g. > from RAID 0 to RAID 5. Perhaps he meant, if you have three drives, > add the fourth to it. > > Douglas, could you clarify?
With a two-drive raid1, if one drive fails, you pull it out and put in a new drive. The array is then sync'd to the new disk. That can put a load on the older disk (which may be the same age, brand, batch even, of the one that failed), which could fail before the sync is completed. With a three-drive raid1, if one drive fails, you still have a two-drive raid1 array to protect the data while a new third drive is sync'd. If you have several raid1 arrays, all with the same drives, you can leave one drive installed as a hot spare for any of the arrays to use in the case of a drive failure. I don't know if the software raid system can handle this automatically or not. I suggested this use of a third drive (if you had it lying around) because of the slowness of raid5. Where raid was really fun was in the older 5U servers with 12 disks on two scsi busses. The drives were only, e.g. 9.6 GB so the more exotic hardware raid types allowed you to create large drives from small drives, present the array to the OS as one large drive and partition it accordingly. This was before LVM. I wish I could have got Etch to boot on the RS/6000 7025-H50; I had a good deal on it. Doug. -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]
