what is tiered storage?

I do not know enough to ask this question better. Searched SF posts but found nothing that could be comprehended.


Tiered Storage is a term you typically hear SAN and NAS vendors talk about. In these storage systems, you typically would have several different tiers of disk in your arrays - some very high-end enterprise SSD or FC disks, all the way down to cheap midline 7200RPM SATA disks. In a setup like this, Tiered storage would allow your data to move around to/from different tiers depending on usage.

One typical instance of this would be a ZFS server with a ZIL (write log) spanning two small mirrored SSDs and then a big bucket of spinning-disk bulk storage. In this instance, all writes would be written at SSD speeds but obviously due to the small size of these SSDs, they can't hold all of your data. As such, the filesystem would migrate data down to the slower/cheaper disk tiers as it is able.

The end result of tiered storage is that it allows you to take advantage of a few fast expensive disks without needing to purchase enough of these disks to hold your whole dataset.

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Tiered storage is storage that has various levels of performance. Generally speaking, it's ranked on the following scale:

  1. SSD. If you really need it fast.
  2. 15K RPM FC/SAS drives.
  3. 10K RPM SAS drives.
  4. 7.2K RPM SAS/SATA drives.
  5. 5.4K RPM drives (uncommon)
  6. Tape or other offline storage.

Some storage systems have most of these elements inside the device itself, which allows a very dynamic allocation of storage across the levels. Other ways of doing it have separate devices for each level and simple management practices allocate storage.

Recently vendors have started promoting dynamic reallocation between storage tiers. NetApp and 3Par currently do this, though in different ways. The 3Par model promotes/demotes storage at the individual block-level between the storage tiers, which allows frequently accessed storage to be fast, and dead storage (8TB of 6 year old office files that get touched once a year if that often) to fall to the bottom even if the fast-access and dead-data are on the same volume. This allows a more efficient use of storage resources.

[Specifically, things like super-blocks can be on SSD which make metadata operations screamingly fast, where unallocated space may be sitting on 7.2K RPM drives]

Tiered storage is the act of having multiple storage tiers available for use. Unfortunately, storage vendors use it to mean the same thing all in one device. It can also be done with discrete devices, though not as efficiently as you could in a single device.

As an example, I have three tiers of storage right now:

  1. 96 spindles of 10K RPM FC drives in a single fibre-channel array.
  2. 48 spindles of 7.2K RPM FATA drives, replicated between two fibre-channel arrays.
  3. 48 spindles of 7.2K RPM SATA drives in an iSCSI array I built from parts.

The difference in the bottom two tiers is cost. Tier 2 is around $13/GB, where Tier 3 is closer to $1.25/GB. Tier 2 can survive a major array failure. Tier 3 just dies in that case.

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