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I could not find anything similar to my question in the forum, but if this is a ‘repost’ I apologize.

I recently took over the IT for a new company and have spent the past 4 months trying to figure out WTF the prior IT guys were smoking. (Basically the entire IT infrastructure is crap, ZERO best practices used… heck I don’t think ANY common sense practices were used in building the network/server/workstation structure) Plus, the IT guy was gone by the time I showed up, so I walked into this blind.

Anyway, long story short, I have a 6TB Synology NAS box. I just recently found that the NAS box is also being used as an iSCSI target. I’m inexperienced with iSCSI so I don’t fully understand the concepts and principles, but what confuses me is, should this box be used as both an iSCSI target AND a NAS box? Reason I’m confused is, each server attached to the box via iSCSI shows very different free space than when connected directly to the NAS using Windows File Sharing.

The iSCSI clients show 4.5TB free, and mapping directly to the box shows less than 2TB free. I know the latter is true, but I don’t understand why the servers show different free/used space.

3 Spice ups
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This is called unified storage. NAS and SAN from the same box is very common.

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SAN and NAS are extremely different conceptually. NAS is a shared filesystem. SAN is remote access to virtual drives. They cannot share resources by definition. So you will not see shared space (NAS) with similar space utilization to your dedicated space (SAN). Unless you are doing some crazy advanced SAN stuff, every machine is going to see it’s “own” SAN space not shared with any other machine.

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With iSCSI, the server is sharing out raw disk space and the client is responsible for file systems, etc.

With NAS, the server is sharing folders, windows-style.

Synology has a “thin-provisioning” feature which allows the box to represent the drive as larger than what’s actually there. This is good for servers where it’s difficult to re-size the drive after it’s deployed, but the growth is slow or at least predictable so you can grow the array later. I think this is just on the iSCSI side, not the NAS side, but don’t hold me to that (EDIT: for Synology I mean, not everywhere).

The NAS side and the iSCSI side are different slices of the array, by the way. Or at least, they really, really should be.

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Here is a “great” video talking about SAN, NAS and some related stuff…

http://www.spiceworks.com/tv/?cat=spiceworld&video=SW-10-open

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With regard to ISCSI, I had my own learning experience when rolling it out for my company, so you are not alone.

While I don’t know the specifics of the Synology NAS box you have, it is conceivable that the 6TB is split up into a few sections and some iscsi clients are logging in to “section #1” and some clients are mapping directly to “section #2”. Therefore, neither is lying to you about free space, they are just looking at a subset of the total storage available.

As you are getting up to speed, I would take my time to understand the terminology that Synology uses and how it applies to doing iSCSI. It is powerful, but can be arcane to use. You will need to get familiar with channels, drives, partitions, initiators, LUN (logical unit numbers), RAID configurations, network topology, jumbo frames (be careful to turn it on - make sure every device and driver supports it, otherwise you may experience errors with lost PDU (Protocol data units) as packet sizes are mismatched)

good luck and ask any other questions you need.

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Michael Z wrote:

While I don’t know the specifics of the Synology NAS box you have, it is conceivable that the 6TB is split up into a few sections and some iscsi clients are logging in to “section #1” and some clients are mapping directly to “section #2”.

Not just conceivable but a base requirement of the NAS and SAN concepts. Things that are one cannot share space with the other. One is block level and one is file level. They cannot share space. The NAS side cannot read the SAN side and vice versa.

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Ok, what you are saying makes sense. Now what I’m concerned/confused about, is if the iSCSI clients and the WIndows clients both see the 6TB array differently, and their total used/free disk space will show differently, what if one is full? Does that mean that the iSCSI clients will still be able to use 6TB if the Windows share is at capacity? Do I technically/virtually have 12TB even though I only physically have 6TB?

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Mike.MDC wrote:

Ok, what you are saying makes sense. Now what I’m concerned/confused about, is if the iSCSI clients and the WIndows clients both see the 6TB array differently, and their total used/free disk space will show differently, what if one is full? Does that mean that the iSCSI clients will still be able to use 6TB if the Windows share is at capacity? Do I technically/virtually have 12TB even though I only physically have 6TB?

Unless there is thin provisioning going on (in which case the space is all fake) then the spaces are unrelated. They do not interact with each other.

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Mike.MDC wrote:

Ok, what you are saying makes sense. Now what I’m concerned/confused about, is if the iSCSI clients and the WIndows clients both see the 6TB array differently, and their total used/free disk space will show differently, what if one is full? Does that mean that the iSCSI clients will still be able to use 6TB if the Windows share is at capacity? Do I technically/virtually have 12TB even though I only physically have 6TB?

Imagine the 6 TB has two LUNs.

The first LUN we designate as iSCSI and share out as raw block i/o. But we tell the server which connects to it that it, by itself is 100TB large (despite being only 3TB large).

The second LUN has 3TB formatted in whatever Synology uses, and is shared as a network drive. It, for the sake of this thought experiment, reports it’s size correctly.

You’re still looking at two different LUNs. LUN A can only grow to 3TB. LUN B can only grow to 3TB. Period.

In order to know what the actual usable drive space on either is, you need to use Synology’s management interface.

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Scott Alan Miller wrote:

I’ve seen thin provisioned iSCSI on boxes. I’d check with the vendor on this.