[linux-lvm] Higher than expected metadata usage?

Zdenek Kabelac zkabelac at redhat.com
Tue Mar 27 10:39:35 UTC 2018

Dne 27.3.2018 v 09:44 Gionatan Danti napsal(a):

> What am I missing? Is the "data%" field a measure of how many data chunks are 
> allocated, or does it even track "how full" are these data chunks? This would 
> benignly explain the observed discrepancy, as a partially-full data chunks can 
> be used to store other data without any new metadata allocation.


I've forget to mention  there is  "thin_ls" tool (comes with 
device-mapper-persistent-data package (with thin_check) - for those who want 
to know precise amount of allocation and what amount of blocks is owned 
exclusively by a single thinLV and what is shared.

It's worth to note - numbers printed by 'lvs' are *JUST* really rough 
estimations of data usage for both  thin_pool & thin_volumes.

Kernel is not maintaining full data-set - only a needed portion of it - and 
since 'detailed' precise evaluation is expensive it's deferred to the tool 

And last but not least comment -  when you pointed out 4MB extent usage - it's 
relatively huge chunk - and if the 'fstrim' wants to succeed - those 4MB 
blocks fitting thin-pool chunks needs to be fully released.

So i.e. if there are some 'sparse' filesystem metadata blocks places - they 
may prevent TRIM to successeed - so while your filesystem may have a lot of 
free space for its data - the actually amount if physically trimmed space can 
be much much smaller.

So beware if the 4MB chunk-size for a thin-pool is good fit here....
The smaller the chunk is - the better change of TRIM there is...
For heavily fragmented XFS even 64K chunks might be a challenge....



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