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

[Gionatan Danti <g.danti@assyoma.it>]

Il 2020-09-09 17:01 Roy Sigurd Karlsbakk ha scritto:
> 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.

It really depends on the use case. I applied it to a fileserver, so 
working at file level was the right choice. For VMs (or big files) it is 
useless, I agree.

> This is all known.

But the only reason to want tiering vs cache is the additional space the 
former provides. If this additional space is so small (compared to the 
combined, total volume space), tiering's advantage shrinks to (almost) 
nothing.

> 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.

IOPs are already well server by LVM cache. So, I genuinely ask: what 
would be tiering advantage here? I'll love to ear a reasonable use case.

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

Yeah, but this need a direct reply from a core LVM developer, which I 
wellcome ;)
Thanks.

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Danti Gionatan
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