[linux-lvm] Higher than expected metadata usage?
g.danti at assyoma.it
Tue Mar 27 11:05:22 UTC 2018
On 27/03/2018 12:39, Zdenek Kabelac wrote:
> 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,
> 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).
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