pereubu
2 Iron

Cx3-80 Newbie question : Raid 5

Jump to solution

Clariion CX3-80 Navispere 6

I have question:

It was cretaed Raid group X [RAID 5;FC]. No private Luns inside RAID group. Overall thee is 6 disks of 66.64GB. There is two LUN overthere:

1st:

User Capacity: 70GB

Raw Capacity: 87.5GB

2nd

User Capacity: 64.55GB

Raw Capacity: 80.694GB

Question 1: RAID 5 will give 248.25GB total usable space. That is not the case. Why? Even if I  bind new LUN there is no free space for it.

Where are around 80 GB?

Question 2: If I unbind one of the LUN (let say LUN 1st), the RAID group will free the DISK space inside RAID group?  Or I have to defragment RAID group?

Labels (1)
1 Solution

Accepted Solutions
Allen Ward
4 Germanium

Re: Cx3-80 Newbie question : Raid 5

Jump to solution

Just to clarify further on the question of defragging a RAID Group... the only time you need to defrag is if you have more "free space" than "contiguous free space". If they show the same value you are fine. I the free space is larger than the contiguous free space than you will need to defrag if you need more than the available contiguous space for your next LUN.

As a best practice in our shop, we always check when we remove a LUN and trigger a defrag immediately if one is required. That way we don't need to wait if we need to do an emergency allocation later.

As already explained, you wouldn't have to worry about it in your described case because the defrag would not gain you anything. But I'll describe a situation in which it could.

If you have a 5 drive RAID Group of 146GB drives you will get roughly 533GB usable if you build RAID 5 LUNs. So we will bind a 5 x 100GB LUNs on this RAID group - LUNs 1,2,3,4 & 5 in order. Later that week the host that LUN 2 was assigned to crashes and needs to be rebuilt from scratch. Before you assign the LUN to the new server the Server Admin tells you that he really needs a single 130GB LUN instead of 100GB. You destroy LUN 2. This leaves you with 133GB free on the RAID Group, but you can still only build a 100GB LUN (because the empty space isn't contiguous). You run a defrag on the RAID Group and when it completes you are able to bind your new LUN2 with 130GB as requested and allocate it to the server.

A picture would probably make it easier, but even if I could draw here I'm no artist. Hopefully the words help enough 🙂

0 Kudos
14 Replies
Highlighted
kenn2347
3 Zinc

Re: Cx3-80 Newbie question : Raid 5

Jump to solution

First question i have is this raid group carved on the vault drives?

If on vault drives, usable space from a 6 disk Raid 5 with 73G drives will be 168G.  If that raid group is not carved on the vault drive, you should get 333G worth of space.

I think your raid group is actually 5 disk raid 5 on the vault drives.  the useable space for that config is 134G and your 2 lun's are consuming that space.

For your second question,  it depends on which lun you unbind first.  look at the properties of the raid group on the partition tab and you can see which lun is first in the group.  if you remove the first, you will have space available before the second lun.  but since you dont have any extra free space in that raid group, you wont get any added space by defraging

Allen Ward
4 Germanium

Re: Cx3-80 Newbie question : Raid 5

Jump to solution

Just to clarify further on the question of defragging a RAID Group... the only time you need to defrag is if you have more "free space" than "contiguous free space". If they show the same value you are fine. I the free space is larger than the contiguous free space than you will need to defrag if you need more than the available contiguous space for your next LUN.

As a best practice in our shop, we always check when we remove a LUN and trigger a defrag immediately if one is required. That way we don't need to wait if we need to do an emergency allocation later.

As already explained, you wouldn't have to worry about it in your described case because the defrag would not gain you anything. But I'll describe a situation in which it could.

If you have a 5 drive RAID Group of 146GB drives you will get roughly 533GB usable if you build RAID 5 LUNs. So we will bind a 5 x 100GB LUNs on this RAID group - LUNs 1,2,3,4 & 5 in order. Later that week the host that LUN 2 was assigned to crashes and needs to be rebuilt from scratch. Before you assign the LUN to the new server the Server Admin tells you that he really needs a single 130GB LUN instead of 100GB. You destroy LUN 2. This leaves you with 133GB free on the RAID Group, but you can still only build a 100GB LUN (because the empty space isn't contiguous). You run a defrag on the RAID Group and when it completes you are able to bind your new LUN2 with 130GB as requested and allocate it to the server.

A picture would probably make it easier, but even if I could draw here I'm no artist. Hopefully the words help enough 🙂

0 Kudos
RRR
5 Osmium

Re: Cx3-80 Newbie question : Raid 5

Jump to solution

if you have 200GB of free space in two equally large areas of 100GB each and you only need to create 1 or 2 100GB LUN's you don't have to defrag at all !

Furthermore: if you have a 100GB "gap" on the left and a 100GB LUN on the right, you could also trigger a LUN migration of that single LUN. This way not all LUN's of that RG will be moved, just that one.

And finally: I wouldn't want to defrag just because you can. The design may be that certain LUN's have to be on the edges of the RG, to keep busy LUN's together in the middle.

The best way to allocate LUN's in a RG for performance reasons is as follows: put the busiest LUN's in the middle and the further you go to the edges, the lower the performance of those LUN's have to be. This way the hard drive's heads will mostly be active in the middle of each disk and therefore perform better !

0 Kudos
pereubu
2 Iron

Re: Cx3-80 Newbie question : Raid 5

Jump to solution

To Kenn2347

That is the case I have bus 0 enclosure 0  disk 0 to 4.

http://clariionblogs.blogspot.com/2008/01/vault-drives.html

I understand  everything you said.

To Allen Ward:

Is it good idea to defrag imeddiately?

Ok,  what I want is to get some disk space free. There is a RAID Group consisting  of 8 LUNs,  3 out of them appaers to be non allocated to any host. There is no any private LUN in the group. These numbers are  3, 4 and 7. Partitioning follows the order of numbers  (First Lun1, Lun2....Lun8).

LUN properties, host tab – no host information, nor Physical Device name neither Logical Device name. So, that means that  particular LUN is not  connected (allocated) to a host(server)?

However, the help is a bit confusing:


Depending on the application you are using, some or all   displayed fields may be read only. Read-only fields appear dimmed and are   unavailable for editing. When a LUN is not visible to any hosts, the   following text displays:

Host information for this LUN is not available.

There is no the above text, and I am confusing. Neither host information  nor LUN Drive mapping for host appear dimmed, but they are empty.

Can I unbind these LUNs, defrag raid group  and bind to it  LUN 3 times capacity of the identical LUN inside the raid group?

0 Kudos
pereubu
2 Iron

Re: Cx3-80 Newbie question : Raid 5

Jump to solution

Do not know what do you mean by trigger migration?

0 Kudos
RRR
5 Osmium

Re: Cx3-80 Newbie question : Raid 5

Jump to solution

LUN migration is used to move a single (or more than 1) LUN('s) to the same or a different Raid Group in order to:

- change a storage tier (move from SATA to FC or vice versa)

- move from a METALUN to a single LUN or vise versa

- clean up a Raid Group instead of having to defrag the whole Raid Group (this only works if there's a LUN at the end of a RG equally sized as the "gap"  somewhere in front or the middle

How do you perform this LUN Migration ?

Create a new LUN in the area where you want to have the LUN migrating to. If you have a 100GB in the beginning of a RG and you want to move a 100GB LUN that's in the back, create a 100GB LUN with any LUN address. Now right click on the LUN you want to move and choose "migrate". Now choose the newly created LUN and voila: the migration will move the data from your original LUN to the new location and in the end the new LUN takes the identity of the old LUN. And the best part is that it's non disruptive !

jps00
3 Argentium

Re: Cx3-80 Newbie question : Raid 5

Jump to solution

Do not forget pools.

LUN migration will move you in and out of pools, and through proper technique can be used to create a move favorable layout of storage within the pool.

Allen Ward
4 Germanium

Re: Cx3-80 Newbie question : Raid 5

Jump to solution

@ RRR

I wasn't considering a special performance consideration for LUNs on a RAID Group. I was answering more from the perspective of the question being asked from someone looking at a basic configuration. You are right though that I should have qualified it so others reading it later would realize that this isn't ALWAYS the right thing to do.

@DoleBlogger

As per the comments from RRR, this isn't ALWAYS the right thing to do, but if you are not configuring your LUNs for special performance based on RAID Group layout then it makes more sense to defrag sooner than later. I can't count the number of times I've tried to provision a LUN for an "emergency" situation and then needed to wait for a defrag to complete before I could get the LUN I needed.

0 Kudos
RRR
5 Osmium

Re: Cx3-80 Newbie question : Raid 5

Jump to solution

so true

0 Kudos