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

[Gionatan Danti]

Il 2020-09-09 21:41 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
>>> 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.
> 
> LVMcache only helps if the cache is there in the first place and IIRC
> it's cleared after a reboot.

I seem to remember that cache is persistent, but a writeback cache must 
be flushed to the underlying disk in case of unclean shutdown. This, 
however, should not empty the cache itself.

> It help won't that much over time with large storage. It also wastes 
> space.

I tend to disagree: with large storage, you really want an hotspot cache 
vs a tiered approach unless:
a) the storage tiers are comparable in size, which is quite rare;
b) the slow storage does some sort of offline compression/deduplication, 
with the faster layer being a landing zone for newly ingested data.

Can you describe a reasonable real-world setup where plain LVM tiering 
would be useful? Again, this is a genuine question: I am interested in 
different storage setup than mine.

Thanks.

-- 
Danti Gionatan
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