EVA8100 diskgroup occupancy alarm best practice

[krs2prs]

Hi,
I just like to know if the 85% best practice for the occupance alarm level is still valid for the new EVA4400, EVA8100, EVA8400? I believe this recommendation was mentioned when we only have EVA3000s and 5000s.
thanks and regards
krs2prs

[Uwe Zessin]

I have never heard about a "85%" best practice. It really depends on the environment. If your setup excludes BC and CA and you make use of the so-called 'protection level' to reserve disk space to recover redundancy from a disk drive failure you can go down to 5GB free space in a disk group.

And the 'occupancy alarm' is just that - a message if the value has been passed. The array continues to work fine.

[Wickedsunny · Accepted solution]

Occupancy alarm level is a percentage of the total disk group capacity in blocks. When the number of blocks in the disk group that contain user data reaches this level, an event code is generated. The alarm level is specified by the user.

Regardless of the EVA product which you are using, its always recommended to keep the Occupancy alarm on. The percentage can be decided by you.

Ideally 85% to 90% is what HP recommends.

Regards,
Sunny

P:S - Rate Solutions with points. It keeps us going..

[Uwe Zessin]

Can you quote an official document?

The latest edition (March 2009) of 4AA2-0914ENW,
HP StorageWorks 4400/6400/8400 Enterprise Virtual Array configuration
Best practices white paper

Says
""* Most misunderstood best practice: 5 GB of free space is sufficient for array optimization in all configurations, or 90 percent/95 percent LUN allocation is a good rule of thumb for free space.

These rules are over simplifications and are correct for only some configurations. Five GB is sufficient only if no snapshots or DR groups exist and some availability tradeoffs are accepted (proactive disk management events). Ninety percent is frequently more than is really required for optimum operation, thus unnecessarily increasing the cost of storage. Be sure to read and follow the free space management best practices.""

The 'free space management' section talks about a mysterious PDM (Proactive disk management) which is an user initiated or automatic - ungrouping of a disk drive from a group.

So in addition to the so-called 'protection level' the user is supposed to keep free the capacity of two more disk drives from the group to be able to do a PDM.

Well, since 2002 I don't recall I have ever seen an EVA do an automatic PDM and have heard about only 1 or 2 cases where a disk drive has been manually ungrouped due to repeated errors reported.

[Wickedsunny]

90% is the default value which is set, once we first go to configure this. I have an extract from the Student guide which says:-

Occupancy alarm level This value is the percentage of total disk capacity used. When the amount of data in the disk group reaches this level, an event code is generated on the SMA or management server. For example, if the capacity of a disk group is 5GB, and the occupancy alarm level is 80%, the event code is generated when the amount of data in the disk group reaches 4GB.
The default value is 90%.

As Uwe said this entirely depends upon the kind of storage configured.

Regards,
Sunny

[krs2prs]

Hi Guys,
Thanks for the replies. I ran across a EVA perforamnce analysis template document before that actually give 85% as one of the checkpoints. I just don;t remeber were that doc is. But I do remember that the template was for eva5000. SO I'm not too sure if that still exists.
thanks,
krs2prs

[Uwe Zessin]

I remember when the EVA was all new the max. occupancy suggestion was rather conservative. The 'at minimum 5GB free in some situations'-rule is only a few years old.

In the early days the EVA sizing was kind of 'black art'. Does anybody remember the formulas with used 'Vfactorx' and secretly assumed a 95% rule? You were supposed to enter the disk dive size in 'Hardware Gigabytes), but a '72GB' disk drive is a little bit larger - I just checked an old, old report and that model had 73.4 HWGB.

Well, it looks like it still is a black art. Some weeks ago I met somebody from another partner who told me they sometimes have to deliver some disks 'for free' after the initial installation, because it turns out his maths does not work.

[McCready]

I think what you are really asking is how much space to leave free on an EVA; personally, I think the alarm should never be a suprise and we should always know how much is left before we think about allocating more.

As to that free space, I would generally lean towards a lower overall percentage free when I have a larger number of drives in a disk group, as that gives the EVA more leeway to do things like split an RSS group, etc. Just from a gut feeling, I would range between 90% in a 56 disk group to 95 percent in a 168 member group.

Of course, as was said, you need to have more space free when you do
- CA replication, for your logs (they are in RAID 1)
- snapshots or snapclones for backups or other purposes
- if you think you'll be removing disks from a group for whatever reason, like upgrading to higher capacity drives
- other work where you tend to keep copies of Vdisks around for a while, or if you want to handle emergency requests for space

[S. Boetticher]

@Uwe: about manual PDM ungrouping:

well, we had that several times on our EVAs, where we got "predictive failure warnings" (like S.M.A.R.T. on standard HDDs) and HP wanted us to proactively ungroup the disk as to not risk a failure (which would require VRaid-Rebuild and risk data loss if another HDD in that RSS set would die)...

OK, a many of that ungroupings were related to the EVA4400 1TB FATA issues with XCS09000xxx, however we even had such things on EVA4000...

[Víctor Cespón]

1 TB FATA drives (NB1000D4450) desesperately need firmware HP06, which should be out today AFAIK.