[linux-lvm] Higher than expected metadata usage?

[Gionatan Danti]

On 27/03/2018 12:39, Zdenek Kabelac wrote:
> Hi
> 
> 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 thin_ls...

Ok, thanks for the remind about "thin_ls" (I often forgot about these 
"minor" but very useful utilities...)

> 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...

Sure, I understand that. Anyway, please note that 4MB chunk size was 
*automatically* chosen by the system during pool creation. It seems to 
me that the default is to constrain the metadata volume to be < 128 MB, 
right?

> For heavily fragmented XFS even 64K chunks might be a challenge....

True, but chunk size *always* is a performance/efficiency tradeoff. 
Making a 64K chunk-sided volume will end with even more fragmentation 
for the underlying disk subsystem. Obviously, if many snapshot are 
expected, a small chunk size is the right choice (CoW filesystem as 
BTRFS and ZFS face similar problems, by the way).

Thanks.

-- 
Danti Gionatan
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