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Efficient Free Space Reclamation in WAFL


October 13, 2017


Ram Kesavan, Rohit Singh, Travis Grusecki, NetApp Inc. Yuvraj Patel, University of Wisconsin-Madison

ACM Transactions on Storage (TOS) Volume 13 Issue 3, September 2017 Article No. 23.

NetApp®WAFL® is a transactional file system that uses the copy-on-write mechanism to support fast write performance and efficient snapshot creation. However, copy-on-write increases the demand on the file system to find free blocks quickly, which makes rapid free space reclamation essential. Inability to find free blocks quickly may impede allocations for incoming writes. Efficiency is also important, because the task of reclaiming free space may consume CPU and other resources at the expense of client operations. In this article, we describe the evolution (over more than a decade) of the WAFL algorithms and data structures for reclaiming space with minimal impact to the overall performance of the storage appliance.


A copy of the paper can be found at:

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