TAILIEUCHUNG - Tracking Back References in a Write-Anywhere File System

Welfare facilities such as refreshments, first aid, foot massage and toilets can be made available even within a small event site. You may want to include entertainment such as music or street theatre for people waiting to start and for any friends/supporters not doing the walk. If you are planning to sell alcohol or have what is known as ‘regulated entertainment’ (which includes music and dancing) you will need to have a licence from the local authority. They will be able to tell you if your venue already has a licence and will give you details of how. | Tracking Back References in a Write-Anywhere File System Peter Macko Harvard University pmacko@ Margo Seltzer Harvard University margo@ Keith A. Smith NetApp Inc. Abstract Many file systems reorganize data on disk for example to defragment storage shrink volumes or migrate data between different classes of storage. Advanced file system features such as snapshots writable clones and deduplication make these tasks complicated as moving a single block may require finding and updating dozens or even hundreds of pointers to it. We present Backlog an efficient implementation of explicit back references to address this problem. Back references are file system meta-data that map physical block numbers to the data objects that use them. We show that by using LSM-Trees and exploiting the write-anywhere behavior of modern file systems such as NetApp R WAFL or btrfs we can maintain back reference meta-data with minimal overhead one extra disk I O per 102 block operations and provide excellent query performance for the common case of queries covering ranges of physically adjacent blocks. 1 Introduction Today s file systems such as WAFL 12 btrfs 5 and ZFS 23 have moved beyond merely providing reliable storage to providing useful services such as snapshots and deduplication. In the presence of these services any data block can be referenced by multiple snapshots multiple files or even multiple offsets within a file. This complicates any operation that must efficiently determine the set of objects referencing a given block for example when updating the pointers to a block that has moved during defragmentation or volume resizing. In this paper we present new file system structures and algorithms to facilitate such dynamic reorganization of file system data in the presence of block sharing. In many problem domains a layer of indirection provides a simple way to relocate objects in memory or on storage without updating any .

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