EVA 5000 - 300 GB Disks - # of Disk Groups

[Gianni_12]

Hi, we have an EVA 5000 with 106 146 GB Disks. They are dividid in two Diskgroups. Each one with 53 disks.

We have boguht 10 300 GB disks, and we planning to ad more in the future. What setup would you recommend for this new 10 disks? Mix them with the 2 existents disk groups? Or create a third disk group?

Ill apreciate your help.

[Ian Vaughan]

Howdy,
Don't put them in a new diskl group or you will lose too much space to "sparing" (equivalent to 1 or 2 disks worth of capacity held in reserve).
If my mental arithmetic is correct you must have at least 8 disk enclusures and hopefully the 2 existing groups are spread over the shelves to give full redundancy (in case a whole shelf fails).
Short answer - add the disks evenly to the 2 existing groups unless particular vdisks in a particular disk group are under pressure for more space.

However, you should ideally have disk groups with number of disks equal to a multiple of 8 or 6 - most efficient use of space in the EVA.

HTH
Ian

[Mark Poeschl_2]

I'd go along with Ian and add 3 to one group and 7 to the other giving you disk groups of 56 and 60 members.

[Gianni_12]

Hi, thanks for your responses. But what about mixing 146 and 300 GB in the same disk group? any disadvantage?

[Urban Petry]

Gianni, very important when mixing disk sizes within the same disk group is the "disk failure protection level" of the disk group. The EVA reserves the space equivalent of the combined size of 2 ("single" protection level) or 4 ("double" protection level) disks in a disk group to compensate for hardware failures. For its calculation it chooses the _biggest_ disks in a disk group, because in a worst-case scenario those disks could fail and therefore enough spare space must be available.
What this means for your situation is, that if your protection level is double and you add 4 300 GB disks to your existing disk group, all of their space (not the physical disks per se, but 1.2 TB space) will be reserved and you gain only 4x 146GB space (the formerly reserved space out of the 146GB-only-diskgroup) - so don't be too disappointed!
In your situation I would decide where to put the disks depending on the future growth: if you expect future growth and don't need evenly distributed disk groups now I would rather add all of them to one disk group and add the next chunk of disks to the other group.
If you create a third disk group, you have the same "space reservation" problems there - plus only a few spindles to distribute the data accesses (which results in lower performance); at least with the current number of disks.

Also as a second-grade goal try to make the number of disks in a disk group dividable by 8 as some others have posted before. This is the number the EVA is most happiest with.

Urban