Question : Problem: RAID 5 - Replacement disk size

I need to create a cheap large storage, using a RocketRAID 1740 PCI 4 port SATA300 card, using 4 drives to create an array of 1.5GB, I am just wondering should one of the drive failed, I could replace it with an identical drive, but as you all know, the models changes very frequently, and the way manufactures calculate their sizes, there can be MBs in differences, so one 500GB drive will show 465GB free while another make may show 462GB free.

If I replaced (for example) the faulty 465GB one with a 462GB model, how would the RAID make up for the missing MBs during rebuilding ?

Answer : Problem: RAID 5 - Replacement disk size

During RAID setup there is the option to use less than the full capacity of the disks so that a smaller replacement will still have enough space. You can define your array as 1.4TB rather than 1.5TB and waste the remaining space to be safe.

From www.hptmac.com/US/productfiles/pdf/RocketRAID%201740%20User%20Manual.pdf :

"The RocketRAID 1740 can create arrays between partitions
on various disks. It is not limited to physical hard disk drives. As a result of this
feature, you have the option of assigning a physical disk to multiple RAID
arrays."

"Multiple arrays can be created using the same set of hard disk drives. The
Capacity option allows you to set aside disk space that be used to create another array, set as a spare disk, or partitioned to act as a single disk (by the operating system)."

So you can create several seperate arrays and have a partition or two acting as a spare "disk", you can even replace two disks with bigger ones and use the spare space at the end of the big ones as a RAID 1 array with RAID 5 on the same spindles. Virtualizing physical disks into partitions this way looks good at first glance but can lead to "hot" disks with twice the amount of I/O on one disk than on another so it is not normally a good idea. You can even make a semi-redundant array by slicing your 4 disks into 10 partitions, making RAID 5 over 9 partitions and having the 10th as spare; that's all well and good to protect against media defects but if a whole disk fails you lose two partitions and so the array breaks!

Don't do it - keep it simple and just waste a bit of space. You can of course use the space you have reserved as another array for temp data knowing that you may have to sacrifice it if a replacement disk is slightly smaller.