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Re: [linux-lvm] some questions



Dne 19.7.2013 20:22, Christoph Anton Mitterer napsal(a):
On Fri, 2013-07-19 at 12:36 +0200, Zdenek Kabelac wrote:
--discards options is for thin volume in thin pool
Is that then really only for thin pools?
I mean not when I "free" the LEs by removing/shrinking an LV, but when
e.g. my ext4 deletes a file and sends a discard... does LVM pass that on
and is that generally affected by --discards?

E.g. in cryptsetup you have a general option which controls whether TRIM
is passed through dm-crypt to lower block layers... I though --discards
of being like that...

By default every normal (i.e. linear) LV should pass-through discards - there is AFAICS no code to block them and you can't disable them.

For thin volume - it's going through the pool first - and you have the choice
either to ignore them completely, process them at pool level or pass-through the pool to pool data device.


  - 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...
Sure... when e.g. using dmcrypt with it would have even some security
drawbacks...

dmcrypt is not a problem here - but when you i.e. lvreduce - then if 'issue_discard' is enabled - then you cannot revert/vgcfgrestore this
metadata operation - since all reduced data would have been discarded.

In general - whenever you create new LV and then you use mkfs - it will discard whole device anyway.

I though it would be enough if the dataalignment offset of the PVs was
correctly to the underlying "blocks". AFAIU they even wouldn't need to
be aligned to the chunksize of the MD, since the only blocks valid for
MD are the ones from the physical drive below (usually 512B or 4KiB) and
in case of striped MD, PAGE_SIZE blocks (usually 4KiB as well) in which
MD reads/writes parity.

Does LVM recommend that the PE's (i.e. the 4 MiB - not KiB) are aligned
to the stripe size? I though the PE size is irrelevant for normal
reads/writes...


Initial extents should be aligned (and ideally all extents) - (lvm2 metadata are located in front of extents)

i.e. my SSD uses 512KiB blocks - so this is the minimal alignment.
it's not always simple to decide which type of striping is ideal.


Zdenek


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