[linux-lvm] Looking ahead - tiering with LVM?

[Roy Sigurd Karlsbakk]

>> I just wonder how it could be possible some day, some year, to make
>> lvm use tiering. I guess this has been debated numerous times before
>> and I found this lvmts project, but it hasn't been updated for eight
>> years or so.
> 
> Hi, having developed and supported file-level form of tiered storage in
> response to a specific customer request, I have the feeling that tiered
> storage (both file and block based) is not so useful as it seems. Let me
> explain why I feel so...

First, filelevel is usually useless. Say you have 50 VMs with Windows server something. A lot of them are bound to have a ton of equal storage in the same areas, but the file size and content will vary over time. With blocklevel tiering, that could work better.

> The key difference between caching and tiering is that the former does
> not increase total available space, while the latter provides as much
> space as available in each storage tier. For example, 1 TB SSD + 10 TB
> HDD can be combined for a 11 TB tiered volume.

This is all known.

> Tiering is useful when the faster volume provides a significant portion
> of the total aggregated sum - which is often not the case. In the
> example above, the SSD only provides a 10% space increase over plain
> caching. You can argue that one can simple enlarge the performane tier,
> for example using a 4 TB SSD + 10 TB HDD, but you are now in the
> ballpark of affording a full-SSD volume - ditching *both* tiering and
> caching.

If you look at IOPS instead of just sequencial speed, you'll see the difference. A set of 10 drives in a RAID-6 will perhaps, maybe, give you 1kIOPS, while a single SSD might give you 50kIOPS or even more. This makes a huge impact.

> That said, LVM already provides the basic building block to provide
> tiering as you can pvmove between block devices. The difficult thing is
> how to determine which block to move, and managing them in an automated
> way.

…which was the reason I asked this question, and which should be quite clear in the original post.

Vennlig hilsen

roy
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Roy Sigurd Karlsbakk
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