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Re: [linux-lvm] some questions



Dne 18.7.2013 20:43, Christoph Anton Mitterer napsal(a):
Hi.

I'd have some questions...

1) Is the --discards option the same than the issue_discards option from
the config file? Since the one defaults to 0 the other to passdown?
If not what's the difference?

Nope

--discards options is for thin volume in thin pool - while lvm.conf option is about discarding free space in VG - in general - unless you are using some
virtual storage for PVs - it's not a wise idea to enable  issue_discards,
since it makes recovery (i.e. going one step back) impossible - what is discarded cannot be recovered...

For thin volumes - it could be used to free already provisioned space from pool (as well as getting native discard on SSD if the pool is on SSD).
Behaviour is configurable.


2) Of an existing LV, can I see how data was actually allocated? And how
can I see the alloc policy of a VG? vgdisplay doesn't show it.

Looks like this attribute should be added to  lvs/vgs.
For now only  lvdisplay shows allocation policy for LV.
To see allocated extents you could use:

lvs -o+seg_pe_ranges


3) Do I see that correctly, when I use Normal as alloc policy,... it
will try to do contiguous allocation... and only do not so,... if not
possible?

Yes


4) md_component_detection and md_chunk_alignment
Will these also work if there is another block layer (e.g. dmcrypt) in
between? Especially the md_chunk_alignment?

it's only one-layer check - there is no device stack analysis.

5) What alignments are needed?
"md_chunk_alignment — If set to 1, and a Physical Volume is placed
directly upon an md device, LVM2 will align its data blocks with the md
device's stripe-width."

What is visible in sysfs  is used.
It's responsibility of each device to provide correct values for alignment.
i.e. if you have crypted  mdraid device - then crypto needs to export
right values.

Zdenek


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