[linux-lvm] Higher than expected metadata usage?

[Zdenek Kabelac]

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.

Regards

Zdenek