Re: xfs metadata overhead

[Danny Shavit <danny@xxxxxxxxxxxxxxxxx>]

I see. We will try this direction.

BTW: I thought that good estimate would be "volume_size - allocated_size - free_space". But it produced quite a difference compared to metadata dump size.

Is there a specific reason?

Thanks,

Danny

On Thu, Jun 23, 2016 at 1:51 AM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:

On Wed, Jun 22, 2016 at 06:58:16PM +0300, Danny Shavit wrote:
> Hi,
>
> We are looking for a method to estimate the size of metadata overhead for a
> given file system.
> We would like to use this value as indicator for the amount of cache memory
> a system for faster operation.
> Are there any counters that are maintained in the on-disk data
> structures like free space for examples?

No.

Right now, you'll need to take a metadump of the filesystem to
measure it. The size of the dump file will be a close indication of
the amount of metadata in the filesystem as it only contains
the filesystem metadata.

In future, querying the rmap will enable us to calculate it on the
fly, (i.e. not requiring the filesystem to be snapshotted/taken off
line to do a metadump).

Cheers,

Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx

--

Regards,

Danny