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

[John Stoffel]

>>>>> "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.