Specifying block group size

Specifying block group size

[Raymond Jennings]

How do I specify the size of a block group at mkfs?

Like, for example, saying that data groups will be 1GiB, but metadata
groups will be 1MiB?

I noticed that they had different default sizes based on the profile
(dup vs single vs raid)

Re: Specifying block group size

[Hugo Mills]

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On Fri, Dec 21, 2018 at 10:18:40PM -0800, Raymond Jennings wrote:
> How do I specify the size of a block group at mkfs?

   You don't -- there's no explicit control over it. The FS will
decide based on the overall size of the filesystem in question.

   Typically, data groups are made of 1 GiB chunks, and metadata
groups are made of 256 MiB chunks (where the RAID level will determine
the number of chunks in a group and the amount of usable space of the
group).

   Hugo.

> 
> Like, for example, saying that data groups will be 1GiB, but metadata
> groups will be 1MiB?
> 
> I noticed that they had different default sizes based on the profile
> (dup vs single vs raid)

-- 
Hugo Mills             | Beware geeks bearing GIFs
hugo@... carfax.org.uk |
http://carfax.org.uk/  |
PGP: E2AB1DE4          |

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Re: Specifying block group size

[Raymond Jennings]

If the filesystem is later resized, does the (default?) chunk size
adjust accordingly?

On Sat, Dec 22, 2018 at 10:04 AM Hugo Mills <hugo@carfax.org.uk> wrote:
>
> On Fri, Dec 21, 2018 at 10:18:40PM -0800, Raymond Jennings wrote:
> > How do I specify the size of a block group at mkfs?
>
>    You don't -- there's no explicit control over it. The FS will
> decide based on the overall size of the filesystem in question.
>
>    Typically, data groups are made of 1 GiB chunks, and metadata
> groups are made of 256 MiB chunks (where the RAID level will determine
> the number of chunks in a group and the amount of usable space of the
> group).
>
>    Hugo.
>
> >
> > Like, for example, saying that data groups will be 1GiB, but metadata
> > groups will be 1MiB?
> >
> > I noticed that they had different default sizes based on the profile
> > (dup vs single vs raid)
>
> --
> Hugo Mills             | Beware geeks bearing GIFs
> hugo@... carfax.org.uk |
> http://carfax.org.uk/  |
> PGP: E2AB1DE4          |

Re: Specifying block group size

[Qu Wenruo]

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On 2018/12/23 上午2:11, Raymond Jennings wrote:
> If the filesystem is later resized, does the (default?) chunk size
> adjust accordingly?

It depends.

Btrfs calculates chunk size based on total rw bytes (no more than 10%)
and clamp it to maximum chunk size (1G for normal data, 10G data for
large fs, 256 for metadata and 1G data for large fs).

So if you're hitting maximum chunk size already, then it will not change.
But if you're hitting 10% limit, then it will change.

Despite that, raid profile and some other factors will also affect the
chunk size.
E.g. RAID5 with 12 disks could break the 1G/10G data chunk size limit.

Thanks,
Qu

> 
> On Sat, Dec 22, 2018 at 10:04 AM Hugo Mills <hugo@carfax.org.uk> wrote:
>>
>> On Fri, Dec 21, 2018 at 10:18:40PM -0800, Raymond Jennings wrote:
>>> How do I specify the size of a block group at mkfs?
>>
>>    You don't -- there's no explicit control over it. The FS will
>> decide based on the overall size of the filesystem in question.
>>
>>    Typically, data groups are made of 1 GiB chunks, and metadata
>> groups are made of 256 MiB chunks (where the RAID level will determine
>> the number of chunks in a group and the amount of usable space of the
>> group).
>>
>>    Hugo.
>>
>>>
>>> Like, for example, saying that data groups will be 1GiB, but metadata
>>> groups will be 1MiB?
>>>
>>> I noticed that they had different default sizes based on the profile
>>> (dup vs single vs raid)
>>
>> --
>> Hugo Mills             | Beware geeks bearing GIFs
>> hugo@... carfax.org.uk |
>> http://carfax.org.uk/  |
>> PGP: E2AB1DE4          |


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