From: Zdenek Kabelac <zkabelac@redhat.com>
To: Gionatan Danti <g.danti@assyoma.it>,
LVM general discussion and development <linux-lvm@redhat.com>
Subject: Re: [linux-lvm] Higher than expected metadata usage?
Date: Tue, 27 Mar 2018 14:52:25 +0200 [thread overview]
Message-ID: <12b56776-daf3-12e3-1847-f381fa52f1d0@redhat.com> (raw)
In-Reply-To: <3aa5c210-13f1-02d7-6fec-79762087325c@assyoma.it>
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
prev parent 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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