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

[Zdenek Kabelac]

Dne 09. 09. 20 v 20:47 John Stoffel napsal(a):
>>>>>> "Gionatan" == Gionatan Danti <g.danti at assyoma.it> writes:
> 
> Gionatan> 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.
> 
> Gionatan> It really depends on the use case. I applied it to a
> Gionatan> fileserver, so working at file level was the right
> Gionatan> choice. For VMs (or big files) it is useless, I agree.
> 
> This assumes you're tiering whole files, not at the per-block level
> though, right?
> 
>>> This is all known.
> 
> Gionatan> But the only reason to want tiering vs cache is the
> Gionatan> additional space the former provides. If this additional
> Gionatan> space is so small (compared to the combined, total volume
> Gionatan> space), tiering's advantage shrinks to (almost) nothing.
> 
> Do you have numbers?  I'm using DM_CACHE on my home NAS server box,
> and it *does* seem to help, but only in certain cases.   I've got a
> 750gb home directory LV with an 80gb lv_cache writethrough cache
> setup.  So it's not great on write heavy loads, but it's good in read
> heavy ones, such as kernel compiles where it does make a difference.
> 
> So it's not only the caching being per-file or per-block, but how the
> actual cache is done?  writeback is faster, but less reliable if you
> crash.  Writethrough is slower, but much more reliable.

Hi

dm-cache (--type cache) is  hotspot cache (most used areas of device)

dm-writecache (--type writecache) is great with write-extensive load (somewhat 
extends your page cache on your NMVe/SSD/persistent-memory)

We were thinking about layering cached above each other - but so far there
was no big demand and also the complexity of solving problem is rising greatly 
-  aka there is no problem to let users to stack cache on top of another cache
on top of 3rd. cache - but what should have when it starts failing...

AFAIK there is no one yet writing driver for combining i.e. SSD + HDD
into a single drive which would be relocating blocks (so you get total size as 
aproximate sum of both devices) - but there is dm-zoned  which solves somewhat 
similar problem - but I've no experience with that...

Zdenek