Sym device

Question

Very basic question .. AFAIK, hyper is a slice on a disk with the size specified. and sym logical device is we create on hypers .. my question is sym device should be minimum of hyper size ? For an instance, if the hyper size is 15GB, the minimum sym dev is 15gb ? or we can have the sym dev of desired size ?

xe2sdc · Accepted Answer

As said before, you CAN create symdevs of almost any desired size. You have limits .. however if you want to create smaller symdevs (forget the term "hyper") you can. The first limit, the easiest to understand, is free space. You need free space if you want to create new symdevs. "symconfigure -freespace list" and "symdisk list" may be of help.

There is another limit: the number of slices on every physical drive. Usually EMC will configure this value to a reasonable value but when you have RAID5 devices you have a huge number of slices on every drive. That's why you have to check this value and the actual number of slices on every drive. If the actual limit is too low, you can raise it up to 255.
Again with "symdisk list" you can easily find how many slices you already have on each disk in the backend. I think that symcfg -v list may give you the limit. Unfortunatly I don't have a box to run tests right now

There are a number of other limits, however they are outside customer control. So it's useless to talk about them here and now. If you fall in some of the other limits you can only open a SR and ask EMC if it's possible to fix your binfile.

-s-

PS a final note .. when configuring small devices, considere using Mirror instead of RAID5. You'll "waste" some space .. but you'll have a better binfile

Message was edited by:
Stefano Del Corno

RRR · Answer

AFAIK a hyper (or a slice) is a slice on each physical disk. A set of hypers make up a symdev. If you need to have a symdev of let's say 18GB, you'll need two 18GB hypers (if you have a RAID5 (3+1) set up, you'd need 4 hypers of 6GB).

I've heard and seen many discussions where people would call a hyper the same thing as a symdev, but IMHO that's because they use 2 way mirrors and the size of 1 hyper is the same as the usable space of the symdev.

RRR · Answer

If you only need a symdev of 2GB, you need (2 way mirror) two hypers of 2GB each. However, you cannot create a single (or more hyypers), you can only create symdevs, so if you need a symdev (2 way mirror) of 2GB, do it in ECC by choosing the symdev size of 2GB or on the CLI by selecting the right number of cylinders.

wishtobeMrEMC · Answer

A set of hypers make up a symdev --> so, if the hyper size is , for instance, 15GB, I can't have a sym dev of < 15GB even if the application requires smaller devices of 2gb. thanks RRR. it helps.

xe2sdc · Answer

When you create logical volumes (or symdevs if you prefer) you choose the size that the host will "see". Enginuity will translate the required size in a set of slices on the disks. If you want a mirrored volume, Enginuity will create two slices of required size. If you want a RAID5 3+1 volume Enginuity will create four slices (3+1), a third each.

You can choose the final size. Enginuity will translate the required size in slices.

Hyper is an old term that with new RAID5 and RAID6 protections have some problems. Maybe we can use it just like symdev.

wishtobeMrEMC · Answer

hyper is something that we create while configuring the dmx itself. for an instance, if i have created 15gb hypers on all disks in a DMX frame, can i still create a sym dev of <15gb on already created hypers ?

Stefano, as per your answer that 'Enginuity will translate the required size in a set of slices' [hypers], it looks like it is possible.. my understanding is disk slice is a hyper and the symdev is a logical volume on hypers .. pls correct me if my understanding is wrong

xe2sdc · Answer

As long as you have free space on your disks (as per "symdisk" output) you can create symdevs.

Once upon a time, "hyper" was a slice of disk ..
When you need a new symdev you pick up hypers (slices of disk) and use them to protect your data. So if you want to create a 2-Way Mir volume you create 2 hypers and "merge" them in a single 2-Way Mir volume. If you want to create a RAID-S (old and unavailable) you create 4 hypers and "merge" them to form 3 RAID-S volumes. As long as you use 2-Way Mir, RAID-S or Unprotected BCVs, each hyper is as big as your desired volume. If you want to create 18608 devices, you have 18608 hypers. As long as you use mirror or RAID-S hyper have a clear meaning.

RAID5 is a brand new and different thing .. When you create RAID5 volumes, you don't need 4 x 18608 hypers to obtain 3 x 18608 devices. When you use RAID5 you don't have hypers .. you have 4 small slices that form a symdev. But you can't convert a RAID5 device into something different, while with 2-way-mir, bcv and RAID-S you can change protection switching from RAID-S to Mir to unprotected.

wishtobeMrEMC · Answer

But in my new working env, we have 32gb hypers on DMX-4 and 16gb hypers on DMX3 and using RAID5. All hypers are already created with the size mentioned. that is why i wanted to know whether i can still create small sym dev of size
< 16gb on hypers created with size of 16gb and 32gb. I dont want to present a 16gb hyper to the host when the requirement is for 2gb or 3gb.

RRR · Answer

Correct. on each physical spindel the hypers, or slices, reside and a logical volume or symdev sits on 1 or more hypers.

RRR · Answer

'symcfg -v list' indeed shows the config info about your DMX. And the max number of slices per spindels is shown there  It's a variable number however, so you could enlarge that on the fly. With 300 and 500GB spindels a number like 128 is very common.

wishtobeMrEMC · Answer

okay cool .. so hyper = slice and logical volume = sym dev. i got it now. thanks

in my previous working env, symdev are already pre-cut as 30gb, 10gb [raid5] and 5gb[raid1] . so, for storage provisioning, I just check for the available free symdev and present them to the host. i never created symdev.

RRR · Answer

so hyper = slice and logical volume = sym devYup

Symmetrix

Sym device