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On my network I have several types of files I want to save on a SAN like:

  1. SQL db's and logs
  2. Exchange data
  3. Random files

Now I'm wondering if I should create one iSCSI Target with a large volume and initiate that from one of the servers. (and share it so other servers can use it too)

Or I should create separate Targets to have each server use its own storage.

For the record the storage could be separated because the servers aren't using the shared data. For one reason I was thinking of one storage is ease of backup. (but perhaps performance could be a problem?)

What would be an advisable configuration for these type of data?

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2 Answers 2

up vote 7 down vote accepted

You are going to need to do separate targets. NTFS is not a cluster filesystem so having multiple boxes writing to the same disk will trash your filesystem or at the very least the files on your filesystem as they will not be aware of what the other systems are doing.

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Hi Zypher. Ok that's a clear answer. I though I had the option to use a share too but if what you say is true then I have no other option! Then I'm wondering about the following If I create my disks for my target and I'm guessing the wrong size for them will I be able to expand them Safely at a later moment? (or is it advisable to make them super big from the beginning so you don't get in to problems?) –  Joost Verdaasdonk Jun 15 '12 at 20:39
    
Depends on what you are using as the iSCSI host, most proprietary SANs do this, and if you are using a Linux based storage box you just need to make sure that you are using LVM for your filesystem management. Other than that you can expand the volumes fairly easily –  Zypher Jun 15 '12 at 20:47
    
Thanks Zypher. To be honest I didn't get the SAN specs from the datacenter yet. For now in my virtual test domain I'm using a virtual iSCSI Target you can download from MS. But thanks for the tips they will surely help. –  Joost Verdaasdonk Jun 15 '12 at 20:53
1  
This is the right answer but for the wrong reason. typically Iscsi volumes stored on NTFS "look" like a single file. Iscsi is a block level protocols (in fact it sends Command Descriptor Blocks) and as such has no idea about filesystems, typically you want separate targets in order to place data at an appropriate storage tier and to allow for differing data availability levels –  Jim B Jun 16 '12 at 0:58
    
@jim, i was talking from the initiator perspective. If you attach more than one host to a single iscsi target that doesnt have a cluster aware filesystem thing break spectacuarly. –  Zypher Jun 16 '12 at 4:04

Reasons for single target

There is one reason: no storage space fragmentation.

No need to plan capacity in advance. As @Zypher points, NTFS is not a clustered filesystem, therefore you will have only one initiator communicate with the target.

You can configure the initiator host as a CIFS host for the other servers to share the storage. Since many database engines such as SQL Sever 2005 refuse to work on network attached storage volumes, your best candidate for the initiator role is the DB host. Exchange and File storage can use CIFS.

Reasons for multiple targets

  • No dependencies between servers

If any single server goes down the rest will work as if nothing happened.

  • No locking latencies or consistency issues

CIFS or NFS cannot provide latency as good as a block storage.

  • Thin provisioning eliminates the fragmentation issue.

Many Disk Arrays nowadays have options such as thin provisioning that makes it easy to let several logical volumes grow into single storage area so that each volume can grow until there is free space left in the storage area without any interference on behalf of the Storage Administrator or host operating system.

  • Storage tiering

Random files are probably not as critical as DB or Exchange for the business and tend to be more difficult to plan capacity for. It might be cheaper to have them on the capacity tiers while it is preferable to have database files on the performance tier unless the database is small and works from memory.

Even without thin provisioning logical volumes can be extended to accommodate the growth of data, but this would require interaction on behalf of the Storage admin and the admins of the servers.

Conclusion

If you can plan capacity in advance or the storage system has thin provisioning option, or storage tiering is important it is advisable to choose multiple targets. If neither of this applies choosing a single target sharing it with CIFS may be a viable option.

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Hi Dmitri thanks for this detailed info this will help me make my choise in a better way! –  Joost Verdaasdonk Jun 17 '12 at 16:35

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