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From: Zdenek Kabelac <>
To: Gionatan Danti <>,
	LVM general discussion and development <>
Subject: Re: [linux-lvm] Higher than expected metadata usage?
Date: Tue, 27 Mar 2018 14:52:25 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

Dne 27.3.2018 v 13:05 Gionatan Danti napsal(a):
> On 27/03/2018 12:39, Zdenek Kabelac wrote:
>> Hi
>> 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?

Yes - on default lvm2 'targets' to fit metadata into this 128MB size.

Obviously there is nothing like  'one size fits all' - so it really the user 
thinks about the use-case and pick better parameters then defaults.

Size 128MB is picked to have metadata that easily fit in RAM.

>> 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).

Yep - the smaller the chunk is - the less 'max' size of data device can be 
supported as there is final number of chunks you can address from maximal 
metadata size which is ~16GB and can't get any bigger.

The bigger the chunk is - the less sharing in snapshot happens, but it gets 
less fragments.



      reply	other threads:[~2018-03-27 12:52 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
2018-03-27 12:52     ` Zdenek Kabelac [this message]

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