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Re: [linux-lvm] Testing the new LVM cache feature



Dne 22.5.2014 12:18, Richard W.M. Jones napsal(a):

I've set up a computer in order to test the new LVM cache feature.  It
has a pair of 2 TB HDDs in RAID 1 configuration, and a 256 GB SSD.
The setup will be used to store large VM disk images in an ext4
filesystem, to be served both locally and over NFS.

Before I start I have some questions about this feature:

(1) Is there a minimum recommended version of LVM or kernel to use?  I
currently have lvm2-2.02.106-1.fc20.x86_64, which mentions LVM cache
in the lvm(8) man page.  I have kernel 3.14.3-200.fc20.x86_64.

With these new targets usually always applies - the newer the kernel and tools are - the better for you.


(2) There is no lvmcache(7) man page in any released version of LVM2.
Was this man page ever created or is lvm(8) the definitive
documentation?

It's now in upstream git as a separate man page (moved from lvm(8))

(3) It looks as if cached LVs cannot be resized:
https://www.redhat.com/archives/lvm-devel/2014-February/msg00119.html
Will this be fixed in future?  Is there any workaround -- perhaps

Yes - cache is still missing a lot of feature - it needs further
integration with tools like  cache_check, cache_repair....

So far it's really only for a preview - I'd not consider to use it
for anything serious yet.

removing the caching layer, resizing the original LV, then recreating
the cache?  I really need to be able to resize LVs :-)

Surely this feature will be implemented.
Meanwhile - you have to drop cache, resize LV, reattach cache...
(drop cache - means to remove cache)

(4) To calculate the size of the cache metadata LV, do I really just
divide by 1000, min 8 MB?  It's that simple?  Doesn't it depend on
dm-cache block size?  Or dm-cache algorithm?  How can I choose block
size and algorithm?

Well this is where your experimenting may begin.
However for now lvm2 doesn't allow you to play with algorithms - the lvchange interface is not yet upstream...

(5) Is there an explicit command for flushing the cache layer back to
the origin LV?

To be developed...

(6) Is the on-disk format stable for future kernel/LVM upgrades?

Well it's still experiemental - so if there will be found some huge problem,
which requires to change/modify format it may happen.

Zdenek


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