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From: Gionatan Danti <>
To: Zdenek Kabelac <>,
	LVM general discussion and development <>
Subject: Re: [linux-lvm] Higher than expected metadata usage?
Date: Tue, 27 Mar 2018 13:05:22 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

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, 

> 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).


Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. -
email: -
GPG public key ID: FF5F32A8

  reply	other threads:[~2018-03-27 11:05 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-03-27  7:44 [linux-lvm] Higher than expected metadata usage? Gionatan Danti
2018-03-27  8:30 ` Zdenek Kabelac
2018-03-27  9:40   ` Gionatan Danti
2018-03-27 10:18     ` Zdenek Kabelac
2018-03-27 10:58       ` Gionatan Danti
2018-03-27 11:06         ` Gionatan Danti
2018-03-27 10:39 ` Zdenek Kabelac
2018-03-27 11:05   ` Gionatan Danti [this message]
2018-03-27 12:52     ` Zdenek Kabelac

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