Re: [PATCH] pg_upgrade: support for btrfs copy-on-write clones

05.10.2013 16:38, Bruce Momjian kirjoitti:
> On Fri, Oct 4, 2013 at 10:42:46PM +0300, Oskari Saarenmaa wrote:
>> Thanks for the offers, but it looks like ZFS doesn't actually implement
>> a similar file level clone operation. See
>> https://github.com/zfsonlinux/zfs/issues/405 for discussion on a feature
>> request for it.
>>
>> ZFS does support cloning entire datasets which seem to be similar to
>> btrfs subvolume snapshots and could be used to set up a new data
>> directory for a new $PGDATA. This would require the original $PGDATA
>> to be a dataset/subvolume of its own and quite a bit different logic
>> (than just another file copy method in pg_upgrade) to initialize the new
>> version's $PGDATA as a snapshot/clone of the original. The way this
>> would work is that the original $PGDATA dataset/subvolume gets cloned to
>> a new location after which we move the files out of the way of the new
>> PG installation and run pg_upgrade in link mode. I'm not sure if
>> there's a good way to integrate this into pg_upgrade or if it's just
>> something that could be documented as a fast way to run pg_upgrade
>> without touching original files.
>>
>> With btrfs tooling the sequence would be something like this:
>>
>> btrfs subvolume snapshot /srv/pg92 /srv/pg93
>> mv /srv/pg93/data /srv/pg93/data92
>> initdb /data/pg93/data
>> pg_upgrade --link \
>> --old-datadir=/data/pg93/data92 \
>> --new-datadir=/data/pg93/data
>
> Does btrfs support file system snapshots? If so, shouldn't people just
> take a snapshot of the old data directory before the ugprade, rather
> than using cloning?

Yeah, it's possible to clone an existing subvolume, but this requires
that $PGDATA is a subvolume of its own and would be a bit difficult to
integrate into existing pg_upgrade scripts.

The BTRFS_IOC_CLONE ioctl operates on file level and can be used to
clone files anywhere in a btrfs filesystem.

/ Oskari