Re: XFS reflink vs ThinLVM

[Carlos Maiolino <cmaiolino@redhat.com>]

Hi Gionatan.

On Mon, Jan 13, 2020 at 12:25:26PM +0100, Gionatan Danti wrote:
> On 13/01/20 12:10, Carlos Maiolino wrote:
> > First of all, I think there is no 'right' answer, but instead, use what best fit
> > you and your environment. As you mentioned, there are PROs and CONS for each
> > different solution.
> > 
> > I use XFS reflink to CoW my Virtual Machines I use for testing. As I know many
> > others do the same, and it works very well, but as you said. It is file-based
> > disk images, opposed to volume-based disk images, used by DM and LVM.man.
> > 
> > About your concern regarding fragmentation... The granularity is not really 4k,
> > as it really depends on the extent sizes. Well, yes, the fundamental granularity
> > is block size, but we basically never allocate a single block...
> > 
> > Also, you can control it by using extent size hints, which will help reduce the
> > fragmentation you are concerned about.
> > Check 'extsize' and 'cowextsize' arguments for mkfs.xfs and xfs_io.
> 
> Hi Carlos, thank you for pointing me to the "cowextsize" option. From what I
> can read, it default to 32 blocks x 4 KB = 128 KB, which is a very
> reasonable granularity for CoW space/fragmentation tradeoff.
> 
> On the other hand, "extsize" seems to apply only to realtime filesystem
> section (which I don't plan to use), right?

I should have mentioned it, my apologies.

'extsize' argument for mkfs.xfs will set the size of the blocks in the RT
section.

Although, the 'extsize' command in xfs_io, will set the extent size hints on any
file of any xfs filesystem (or filesystem supporting FS_IOC_FSSETXATTR).

Notice you can use xfs_io extsize to set the extent size hint to a directory,
and all files under the directory will inherit the same extent hint.

Cheers.

> 
> Thanks.
> 
> -- 
> Danti Gionatan
> Supporto Tecnico
> Assyoma S.r.l. - www.assyoma.it
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> 

-- 
Carlos