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First of all, I need to point out that I ain’t work for EMC nor VMWARE. I won’t do anything like Chad in Virtual Geek to say, “Yes, it’s true. EMC really IS #1 for VMware.” What I’m going to to talk about is purely from a customer point of view. A tech who doesn’t favorite either EMC nor Netapps. I just want to draw a picture of EMC storage and Vmware related technologies in front of you so we can put everything on the table and discuss it.

As usual, any comments and discussions are very welcome here.

EMC Storage – Celerra, CLARiiON and Celerra Unifed Storage

Well, back in years ago, There are three production lines in the EMC storage. Celerra, CLARiiON and Symmetrix. After years of Virtualization development, the line between Celerra and CLARiiON is getting really blurry. Celerra used to dedicate to NAS file system. It provides NAS only. With CLARiiON(like CX3 series), it mainly focus on Block level storage like FC or iSCSI. Now, with new product line Celerra Unified storage coming out, I don’t really think anyone would buy old system any more. Because new Unified Storage provides both Celerra & CLARiiON in one box. EMC call them block enabled Celerra system (NS-120,480,960,etc). However, as you may know the technology using in NAS are quite different from technology using in the block storage. If you are as confused as me, please read rest of article and hope it can help you clear your mind. This article is focusing on Celerra Unified storage only.

EMC Deduplication and Compression

As everyone knows, one of key elements of storage is disk capacity. How to utilize disk space and tier down unused data and files and compress them to small space becomes the major reason when we select Storage. Even after years and years research, EMC still insists deduplication should only happens on file level instead of block level. so what does that mean to customers who bought Celerra Unified Storage? It means you can only use block compression if you use block storage (like FC, iSCSI, this is as part of CLARiiON technology) and you can only use file level Compression and file level deduplication when you use NAS (as part of Celerra, if you use NFS for VM or NFS,CIFS for file systems). In other word, how  you divide your LUNs and what kind of block or file system you use will dramatically impact your system. Let’s break down those technology and see what they are.

EMC compression

As what I mentioned above, depends on what kind of system (NAS or block) you use, Celerra will use different ways to compress the data. Let’s talk about block level compression first.

Block level compression

As what this name indicates, this compression should only work on FC or iSCSI LUNs. The block size compression works on is 64KB. Each block is processed independtly. The typical result of compression is as much as 2x while it use modest CPU resource.


In default, CX4-480 can have 8 concurrent threads on compression. When all threads running at same time, the consumption of CPU will be compression rate(speed) as Low (15% CPU), medium (30~50% CPU) and high(60%~80% CPU).

How block compression works

1.Initial compression- This occurs when you enable compression. It compress entire LUN and can’t be paused in the middle. But it can be disabled during the procedure. No damage will be done.

2. Compression of new data – When new data is written, it is written uncompressed and compressed asynchronously. It keeps doing that until you disable compression. In default, when 10% of user capacity of LUN or 10GB new data are written, and total amount new data is larger than 1GB, compress starts. It does use some of SP cache memory for swapping. When you compress a LUN, that LUN will automatically migrate to a thin LUN in different pool if LUN is a normal RAID lun. If it’s a think LUN, it will reminds in the same pool.

3. Decompression when compression is disabled- if the original LUN is a thin LUN, it will reminds thin LUN. If the original LUN is a thick LUN or RAID group LUN, it will write zeros to unallocated capacity till full while it reminds a thin LUN. System will pause at 90% and stop at 98% if the LUN has filled up too much.

Limits of block compression

  • The following cannot be compressed:
  • Private LUNs (including write intent logs, clone private LUNs, reserved LUNs, meta LUNs, and component LUNs)
  • Snapshot LUNs
  • Celerra iSCSI or file system LUNs ( Personally, I don’t think that’s right. I’m confirming with EMC now)
  • A LUN is already being migrated and expanding or shrinking.
  • A mirrored LUN replicating to a storage system running pre-29 FLARE code.

Interactive of compression with other functionalities

Basically, a compressed LUN is transparent to other operations like replication or migration. But by saying that, it’s better not migrating or copying while compress at same time. It’s always easy for SAN to enable compress after migration.

How to setup compression?

All what you need to do is to connect Celerra Unified storage control station with your Internet Browser. You will have Unisphere Manager running directly from SAN or you can install Unified Manager on a windows server and connect to your box. Compression function is a licensed feature and you should have it directly from console. Unlike Celerra NAS part, there is no VMWARE plug-in availabe for compression so you need to use Unisphere to do the job.

There is no Vmware plug-in?

It is very interesting that Celerra Unified NAS part got a vmware plug-in while CLARiiON reminds nothing. I reckon vSphere may use VAAI API to offload clone from host to SAN but why it doesn’t work for Celerra NAS part? If anyone can answer this question, it will be appreciated.

To be continued……



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  1. […] GeekSilver's Blog About « EMC Celerra Unified, CLARiiON storage with Vmware – Deduplication and Compression Part 1 […]

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