TarMK and SAN

Yesterday’s posting „TarMK and NAS“ got quite some attention, and today I got several times the question „And what about SAN?“. Well, here the answer to „Do you recommend TarMK on SAN?”

A little background first: A SAN (Storage area network) is a service, which offers block devices. It is part of today’s enterprise datacenter’s infrastructure, where attaching local disks to servers is not feasible and does not scale. If you are familiar with PCs, you can consider a block device like a partition on your hard disk. You cannot use a partition by itself, but you have to format it and put a filesystem on it.
That’s basically the same with a SAN: You get it as a raw device (called volume), you put a filesystem on it (for example ext4 when you use Linux) and then you can use it just like any other local drive. The only difference to a local drive is, that the connectivity is not provided by local SATA-port, but over a network (you’ll find the terms iSCSI or Fiber Channel, but that’s too much detail here).

And that’s the huge difference: With a SAN you get a block device, with a NAS you’ll get a shared filesystem.

So the basic principle is, that you can treat SAN like any local storage. And if you format it with a filesystem like ext4, btrfs or NTFS on Windows, you have a local filesystem. And like I said in yesterday’s post: When you have a local filesystem, where only a single system is controlling access to it, you can use mmap. And mmap is all we care about here!

My recommendations for TarMK are:

  • When you have the choice between SAN and NAS (sometimes you have): drop the NAS and go for SAN.
  • And when you have the choice between SAN and local drives, choose „SAN“ as well. Why? Because you never need to deal with the problems of „my hard drives are full and we don’t have any empty drive bays anymore on this server!“ anymore. Just allocate some more space to your SAN volume, resize the filesystem and that’s it. When you have mmap available for your TarMK, filesystem performance shouldn’t be something to worry about.

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