spare policy of single

[Fauziah Mahdan]

Hi all, I have EVA6000 SAN Storage installed with 28 disk.
I was informed 2 har disk was used for spare policy of single. What is this spare policy of single means?

[Uwe Zessin]

There are 3 levels: none (0), single (1) and double (2). The EVA does not have dedicated spare disk drives. Instead, the capacity is 'virtualized', too, in the disk group.

Level none does not reserve any capacity, but the EVA can still rebuild data if there is enough unused capacity in the disk group.

Level one reserves two times the capacity of the largest disk drive in a group. For example: if you build a group with 300 and 600GB disk drives, then 1200GB is reserved.

Why 2x?
The EVA always 'marries' disk drives in pairs. Such a pair defines where VRAID-1 data is stored. If you have a disk group with an even number of disk drives (e.g. 8, 10, ...) and one disk fails, the surviving partner can no longer be used to store VRAID-1 data.

The EVA recovers redundancy by taking the data from that disk and moving it to other disk pairs. If the disk group is completely filled with VRAID-1 data and has an odd number of disks (9,11,13,...), then one disk drive is unused, because it is missing a 'partner' to store VRAID-1 data on.

Level double reserves 4x the capacity, but it is mainly intended for EVAs that go unmonitored for long periods. In most cases level single is sufficient.