topic Re: EVA // Remaining space & Allocation Alarm Level in Disk Enclosures

EVA // Remaining space & Allocation Alarm Level

arkie — Fri, 02 Jul 2010 10:23:06 GMT

Hi All,

I have a basic query on EVA space. Pls help me understand this.

Let us consider an EVA 4000 2C4D array with 56 x 300 GB disks. Allocation Alarm Level in the Default Disk Group (the only Disk Group) is 95%.

In this case,

Raw capacity = 56 x 300 GB = 16800 GB
Formatted capacity = 56 x 279.39 GB = 15645.84 GB
In StorageWorks CvEVA > EVA Storage Network Properties, under Managed Storage System Capacity,
Total = 15083 GB <- where's 562.84 GB
Suppose 13736 GB is already allocated
Then, remaining 1346 GB <- must be Vraid0

If it is Vraid5, maximum size of the Vdisk is 1076 GB (which is about 80% of Vraid0 disk)

My concern is - without triggering the Allocation Alarm, what is the maximum size that we can allocate to a new Vdisk. Here's my workout:-

95% of 15083 GB = 14328.85 GB
Already allocated (in this case) = 13736 GB

So, max allowable Vraid0 disk size = (14328.85 - 13736) = 592.85 GB
If we want a new Vraid5 disk, its maximum capacity can be 80% of 592.85 GB = 474.28 GB

Pls correct me if I am wrong.

Re: EVA // Remaining space & Allocation Alarm Level

Víctor Cespón — Fri, 02 Jul 2010 10:42:08 GMT

You have "Disk drive failure protection" set to "Single". This reserves the space equivalent to two phisical disks (2 x 279.39 GB). This space is reserved to perform rebuild in case a disk fails and cannot be used for vdisks.

So you have (56 - 2)x 279.39 GB = 15087 GB available for vdisks. 95% of that is 14332.

A vdisk in RAID 5 takes a 25% more space than in RAID 0 (EVA uses 4 data + 1 parity).

So your calculations are correct, yes.

Re: EVA // Remaining space & Allocation Alarm Level

arkie — Fri, 02 Jul 2010 11:03:44 GMT

Thanks Vcespon for your immediate and informative response

Can you also pls let me know if disk space reservation in case of Disk Drive Failure Protection set to None & Double ... are 0 and 4 respectively.

EVA uses 4 data + 1 parity << pls clarify this part

Re: EVA // Remaining space & Allocation Alarm Level

Uwe Zessin — Fri, 02 Jul 2010 11:21:00 GMT

Right, protection level NONE(or 0) does not reserve any space, but the EVA is still able to reconstruct redundancy if there is enough free space in the disk group. I would still not use NONE, because this prevents one from miscalculation.
And level double does indeed reserves 4 times the size of the largest disk drive in the group.

4D+1P indicates how the EVA does VRAID-5 redundancy. It takes 4 Data chunks, calculates the Parity chunk and stores all five chunks on five different disk drives in the group so that any one disk drive can fail, but you do not have lost data.

Well, at the low level it is a bit more complex, but the basic rule is 4D+1P on five different physical disk drives.

Do not confuse the "5" in VRAID-5 with the "5" in "five disk drives". They are not related.
Other arrays do a RAID-5 with 3D+1P or 7D+1P or other values.

Re: EVA // Remaining space & Allocation Alarm Level

arkie — Fri, 02 Jul 2010 12:04:14 GMT

Thanks Uwe, for the inputs & clarification