Dne 09. 09. 20 v 20:47 John Stoffel napsal(a):
"Gionatan" == Gionatan Danti <g.danti@xxxxxxxxxx> 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
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