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Durring an install of CentOS 6.4, I see two locations where I can select "Allowable Drives"

The first time I select "Allowable Drives" is to select which drives are added to my fstab. That makes sense, and I have no questions there.

The second time I see "Allowable Drives" is while creating a partition. enter image description here

According to the description

Allowable Drives: This field contains a list of the hard disks installed on your system. If a hard disk's box is highlighted, then a desired partition can be created on that hard disk. If the box is not checked, then the partition will never be created on that hard disk. By using different checkbox settings, you can have Disk Druid place partitions where you need them, or let Disk Druid decide where partitions should go. http://www.centos.org/docs/5/html/5.2/Installation_Guide/ch04s18s05.html

I am adding an ext4 partition to disk sdb. I have gone through the partitioning several times, experimenting with checking sdb, sdb and sdd, and just sdd

So far, the partition is created correctly regardless which drive is checked in this box.

Can someone enlighten me what these options are supposed to change?

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up vote 2 down vote accepted

The system will select the first drive which has enough space to accommodate your partitioning request. If you had tried to create a partition of greater size than the available space on sdb, it would have selected sdd (assuming sdd had sufficient space) automatically. If you select a partition larger than sdb can handle and don't allow sdd, it won't let you create the partition. In other words, it does exactly what the description you posted says.

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Thanks for explaining that. I had completely overlooked the fact that a partition can span multiple drives. It makes perfect sense now. – spuder Jul 23 '13 at 18:17
A partition cannot span multiple drives. The behavior is still the same, though - if you attempt to create a 40G partition above, it will go on sdb. If you attempt to create a 90G partition, it will go on sdd. – John Jul 23 '13 at 18:23
That makes sense. I'm not an expert, but I believe a partition can technically span multiple drives if you are using lvm. (not applicable in this case) – spuder Jul 23 '13 at 19:00
A partition cannot. An LVM volume group or logical volume can. Each partition (or whole-disk device) is a PV, one or more PVs make a VG, each VG contains 0 or more LVs, each LV is zero or one filesystems. – John Jul 23 '13 at 19:14

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