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Lightweight metadata timestamp validation

IP.com Disclosure Number: IPCOM000202158D
Publication Date: 2010-Dec-06
Document File: 1 page(s) / 23K

Publishing Venue

The IP.com Prior Art Database

Abstract

The AIX (Advanced Interactive Executive) LVM (Logical Volume Manager) supports three different formats for representing volume group metadata persistently on disk. These are oldvg, bigvg, and svg (Scalable Volume Group) formats. The VGDA (Volume Group Descriptor Area) for oldvg and bigvg metadata formats has a header and trailer timestamp. To validate the metadata copy on each disk when the volume group is brought online (using the varyonvg command), the volume manager compares the header and trailer timestamps and if they are not equal the metadata copy on that disk is considered invalid. The volume manager discards the invalid copies of metadata, then compares the timestamps for the remaining metadata copies and uses a metadata copy with the latest timestamp.

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Lightweight metadata timestamp validation

The AIX® (Advanced Interactive Executive) LVM (Logical Volume Manager) supports three different formats for representing volume group metadata persistently on disk. These are oldvg, bigvg, and svg (Scalable Volume Group) formats. The VGDA (Volume Group Descriptor Area) for oldvg and bigvg metadata formats has a header and trailer timestamp. To validate the metadata copy on each disk when the volume group is brought online (using the varyonvg command), the volume manager compares the header and trailer timestamps and if they are not equal the metadata copy on that disk is considered invalid. The volume manager discards the invalid copies of metadata, then compares the timestamps for the remaining metadata copies and uses a metadata copy with the latest timestamp.

Because the metadata for the scalable volume group metadata format is much larger than oldvg or bigvg, the volume group descriptor area is divided into five distinct parts which can each be written separately. This granularity helps avoid unnecessary I/O operations. For example, if one logical volume control block needs to be written then the volume manager only needs to perform I/O to the region that describes logical volume control blocks, without having to update the other four regions that describe unrelated parts of the volume group descriptor area.

When validating volume group descriptor area timestamps for oldvg or bigvg metadata formats, the volume manager only needs to read two timestamps and perform one comparison in order to determine whether a particular disk contains valid metadata. One problem with breaking up the scalable volume group metadata format into five regions that are able to be written separately, is that if there were a header and trailer timestamp on each area then ten timestamps reads per disk and nine comparisons per disk would be ne...