I recently built and implemented a VTL (Virtual Tape Library). Database backup files were placed on a disk drive. Those files were continuously copied to tape, retained on-site until the next weekly tape pickup, and they were then moved off-site. Another team dealt with the tape copies, off-site moves, etc. That left me to deal with the disk portion, and that’s all my software addressed. It would make full backups, incremental/differential backups, and transaction log backups to disk based on a schedule. Retention of each type of backup is configurable for each server. Restores are always from disk. If the disk backups have aged past the retention period, or were otherwise corrupted, tapes would need to be recalled on-site and restored to the VTL directory… in essence staging those files for the database restore. It’s a very reliable and inexpensive backup method… but what does all that have to do with SAN VS NAS?

I was recently at the Percona Live 2015 conference talking about the process. The subject turned toward the type of disk storage used. Looking at the pros & cons of each type of shared storage was pretty easy for this specific applications. As we were discussing pros and cons of each, one guy asked what the difference was between SAN and SAS. Everyone had their own opinion that went above and beyond the basic difference. Things like SAN is more expensive than NAS. NAS is slower. Both of those are not necessarily true. So, what is the difference between SAN and NAS?

SAN stands for Storage Area Network. NAS stands for Network Attached Storage. That really does not help much; I added it for completeness. The core difference between SAN and NAS is SAN uses block level protocols to access the data, whereas NAS uses file level protocols to access the data. One can think of it in a MS Windows(tm) desktop environment as Shares are similar to NAS and Drives are similar to SAN (although the physical storage is external to the desktop). Also note, the storage can be a hybrid. For example, the open source product: openfiler This device will function with many file and/or block level protocols. Try to deduce the difference between SAN and NAS protocols from their Products page (tab) 🙂 SAN seems to be talked about explicitly, but NAS information is mostly implied.

So, which makes more sense for the VTL: SAN or NAS? I vote for NAS as the only storage operations are file-save/write, file-sequential-read, and file-delete. If we were talking a storage container for a database engine, I’d suggest that SAN would be more appropriate as the file is read & written at the block level.

Troy Frericks.
blog 12-May-2016
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