From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Wysochanski Date: Wed, 23 Jun 2010 09:50:47 -0400 Subject: Concerns about metadata balance downgrading / backwards compatibility Message-ID: <1277301047.9289.18.camel@f10-node1> List-Id: To: lvm-devel@redhat.com MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit In review of the metadata balance patches, concern was raised about backward compatibility. In particular, the following questions were raised: Q. How does the user "go back" to a previous revision of lvm if he's been using metadata balancing? A: If the user wants to use an old version of the lvm tools, he SHOULD use "vgchange --vgmetadatacopies N", where N >= vg_mda_count (number of mdas across all PVs in the VG). The reason this is recommended is so that all mdas contain the latest version of the metadata. If he does not do this, when using the old tools, it will still work, but he will get a warning that the VG is inconsistent. Even in this case though, the next time the metadata is written, it will be written out consistently, with a message stating the older copies of metadata have been updated. Q: Won't the 'ignore' bits in the mda_header and/or the "metadata_copies" value written in the VG metadata cause problems with older LVM tools? A: No. The placement of the 'ignore' bits was done in a location on disk in the mda_header that was previously a 'filler' field (see patch #1). As such, the value of the field is irrelevant to previous LVM versions. As far as the VG metadata, LVM is written to disregard fields it does not understand in the VG metadata, so older tools will not parse the "metadata_copies" field/value, and will not write it back out the next time an LVM operation updates the metadata. I think this addresses the compatibility concerns, without need for further patches. If there are other specific concerns, please let me know. Thanks.