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This question is a follow up to this.

We have a Windows Server 2008 R2 domain over a network that spans two different buildings, linked by a 100-Mbps point-to-point line. Over 60 users work in the organisation. We are planning to use DFS folders and DFS replication for file serving across the organisation. The estimated data volume is over 2 TB, and will grow at approximately 20% annually. The idea is to set up a DFS file server in each building and use DFS so that all the contents stay replicated over the 100-Mbps link.

We are now considering backup hardware and strategies. We are Dell customers and, after browsing the online Dell catalogue, I can see a number of backup hardware options. My main doubts are the following:

  1. Would you go for a tape library, disk backup, or are there other options worth considering?
  2. Would you perform batch backups (i.e. nightly) or would you use continuous backup (i.e. while users are working)?
  3. Would you use a dedicated backup server to which the tape library (or any other backup device) is attached, or is there any other alternative way of doing things?

My experience with backup hardware and overall setup is limited, so I appreciate any good piece of advice that you may have. Thanks.

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up vote 2 down vote accepted

First, I would consider a different structure for the solution. You want to avoid the situation where two users access the two versions of a file at the same time and make conflicting changes. DFS Replication can deal with this, but it isn't pretty and can require administrator intervention. I suggest one primary location for sharing files, replicated to the other location for backup. You have a huge pipe, so unless the files are large it should perform well. Or, each location hosts files used mostly at that location, and those files are replicated across the network for backup.

DFS Replication is pretty good, but does require care and feeding, and does leave you open to update conflicts. Users will have to learn to not leave files open .. they need to get in, make changes, and get out. Another alternative is to use DFS Replication but migrate highly used files to a SharePoint document library, which can manage locking and version control.

Regarding backup ... I like tape .. for historical archives if nothing else. The replication mitigates against data loss from hardware failure, fire, etc. So the questions become:
- What scenarios are you trying to mitigate?
- What impact from these scenarios is tolerable?
- How much can you spend?

Those answers determine what must be backed up and how often, which in turn allows you to define what you need. The solution might be a tape library, disk-to-disk-to-tape, a SAN that replicates itself each night, nightly backup to Amazon S3, a SunGard Backup Facility, Iron Mountain Online Backup, some combination of these, or something else entirely.

If you need somewhere to start, I would put a tape library in one location, backup everything to tape each night, and use an off-site tape service every day. I would not worry about server recovery as much as making sure that all of the user data is saved every night.

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Many thanks for all the info. Most useful. Given your advice on DFS Replication, would it make sense to use a single file server as writeable server and another one (using DFS Replication) as a read-only machine for backup purposes? In that case, I guess we could attach the tape library to this machine, right? – CesarGon Mar 11 '10 at 19:33
Regarding your questions, the scenarios I am trying to mitigate involve disaster recovery (fire, floods, theft) and data loss (user deleting a folder by accident). One day old backups is good enough for most users; we don't need finer resolution. We can easily spend 5,000 €, maybe 10,000 and hardly 20,000 on the backup solution. – CesarGon Mar 11 '10 at 19:45
Your bandwidth should allow a single primary server with a second as backup. Certainly you can the tape library and backup software on the secondary server and do a complete backup daily. This should be sufficient for the conditions you describe at a reasonable cost. – tomjedrz Mar 12 '10 at 5:18
If you have frequent "please restore the stuff file I deleted by accident" requests you might want to do disk-to-disk-to-tape in order to have a backup or two available online. – tomjedrz Mar 12 '10 at 5:21

As tomjedrz says in his answer, DFS replication can be problematic with files being modified in multiple locations simultaneously. If your data is such that the modifications for a given set of files only occur in one building (i.e. on one server) the majority of the time then you may not have problems with DFS replication and conflicts.

Using a single file server to share the files mitigates the possibility of conflict. The 100Mb/sec pipe is certainly not slow, but depending on the size of the files being shared and frequency of access the users in the building w/o the file server may be subject to delays opening files. You'll have to benchmark that and see. Microsoft Office documents over a 100Mb/sec pipe are no problem. Multi-gigabyte solid model files over a 100Mb/sec pipe may be a source of major pain for the users.

To summarize: I think DFS replication is definitely an option, but you'll have to gauge the "character" of your file modification and access patterns to decide if it's a "win". A single file server can be a "win", too, but might also be a "loss" for some types of file access in the "remote" building where that single server isn't located.

DFS replication only protects you from catastrophic failure / destruction of the replica. Accidental deletion or data corruption will require some other type of protection. Additionally, if your buildings are situated closely, physically, I'd argue that having a replica in the other building might not be sufficient defense against physical disaster (fire, flood, theft, etc). You'll have to gauge the risk and act appropriately.

Volume Shadow Copy might just be the ticket for accidental deletion / corruption protection. I haven't used it extensively in production, but it certainly seems reasonable and has performed properly in lab settings. (Just don't be fooled into believing the marketing hype that tries to characterize it as a true versioned filesystem and you'll be fine...)

You would still need some type of off-site and offline backup (ideally with muliple full back retention capability). Tape probably fits the bill from a cost / performance standpoint.

An LTO-4 autoloader would make short work of backup of that data, so long as your server computers can feed data to the drive fast enough. Whether or not you run full backups every day will depend on your backup window and the speed of the backup. The cost per tape is pretty reasonable (around $50.00, last time I looked) such that you can maintain a rotating archive of older backups at a manageable expense.

Hopefully you can time things such that you'll exceed your backup window or the capacity of the autoloader at the end of the planned lifetime for the autoloader. At 20% / year growth, your 2TB volume should be around 3.5TB at the end of year 3. An 8 slot LTO-4 autoloader (800GB natve / 1.6TB compressed capacity per cartridge) would still have capacity. You might have to move to full / differential backups to continue to fit in your backup window, but it seems feasible to get 3 years out of an LTO-4 autoloader solution.

I agree w/ tomjedrz, too, re: the possibility of doing disk-to-disk-to-tape if you have frequent needs for performing incidental restores (and either aren't using Volume Shadow Copy or aren't getting the protection you need out of it). Something simple like a RAID-0 or RAID-10 array of inexpensive SATA disks can feed data to the LTO-4 fast enough to keep it running and can serve as a place to perform quick incidental restores from w/o having to deal with mounting tapes (going to the off-site storage facility to get tapes, etc).

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Many thanks, Evan. I like the idea of disk-to-disk-to-tape backups. The thing is, we will have at least one large (2.7 TB) file server, one database server, and other minor servers (domain controller, for instance) that need backing up. How can we channel data from all these servers into the RAID-0 array that will feed data to the LTO4 autoloader? What kind of hardware/software combination would you use for this? Thanks. – CesarGon Mar 26 '10 at 8:59

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