Journaling vs Soft Updates: Asynchronous Metadata Protection in File Systems Margo I. Seltzer, Harvard, Gregory R. Ganger CMU, M. Kirk McKusick, Keith A. Smith, Harvard, Craig A.N. Soules CMU, Christopher A. Stein, Harvard Presenters: Arjumand Younus and Muhammad Atif Qureshi 1

Outline Introduction: The Problem Solutions – Synchronous Writes – Soft Updates – Journaling Comparative Evaluation – Feature Comparison – Measurements Conclusion 2

Problem Statement File system meta-data update problem – Interdependencies must be cared for during disk updates 3

Metadata Operations Metadata operations modify the structure of the file system – Creating, deleting or renaming files, directories or special files Data must be written to disk in such a way that the file system can be recovered to a consistent state after a system crash 4

Deleting a File (1/3) 5 abc def ghi i-node-1 i-node-2 i-node-3 Assume we want to delete file “def”

Deleting a File (2/3) 6 abc def ghi i-node-1 i-node-3 ? Cannot delete i-node before directory entry “def”

Deleting a File (3/3) Correct sequence is 1.Write to disk directory block containing deleted directory entry “def” 2.Write to disk i-node block containing deleted i- node Leaves the file system in a consistent state 7

Creating a File (1/3) 8 abc ghi i-node-1 i-node-3 Assume we want to create new file “tuv”

Creating a File (2/3) 9 abc ghi tuv i-node-1 i-node-3 ? Cannot write directory entry “tuv” before i-node

Creating a File (3/3) Correct sequence is 1.Write to disk i-node block containing new i-node 2.Write to disk directory block containing new directory entry Leaves the file system in a consistent state 10

Approaches to Metadata Management Synchronous Writes – FFS Ordered Writes – Soft Updats Logged Writes – Journaling 11

12 Synchronous Writes Soft Updates Journaling – LFFS-file – LFFS-wafs

Synchronous Writes Used by FFS to guarantee consistency of metadata: – All metadata updates are done through blocking writes Increases the cost of metadata updates Can significantly impact the performance of whole file system 13

Soft Updates Uses delayed writes (write back) Maintain dependency information about cached pieces of metadata: – This i-node must be updated before/after this directory entry Guarantees that metadata blocks are written to disk in the required order 14

Problems in Soft Updates (1/2) Soft Updates guarantee that file system will recover into a consistent state but not necessarily the most recent one – Some updates could be lost Cyclical dependencies: – Same directory block contains entries to be created and entries to be deleted – These entries point to i-nodes in the same block 15

Problems in Soft Updates (2/2) 16 i-node-2def NEW xyz NEW i-node Block A Block B We want to delete file “def”and create new file “xyz”  Cannot write block A before block B: Block A contains a new directory entry pointing to block B  Cannot write block B before block A: Block A contains a deleted directory entry pointing to block B

Solution to Soft Update Problem (1/2) Roll back metadata in one of the blocks to an earlier, safe state (Safe state does not contain new directory entry) 17 Block A def

Solution to Soft Update Problem (2/2) Write first block with metadata that were rolled back (block A’ of example) Write blocks that can be written after first block has been written (block B of example) Roll forward block that was rolled back Write that block Breaks the cyclical dependency but must now write twice block A 18

Journaling Logs metadata operations Writes metadata in-place asynchronously Write-ahead logging (WAL) protocol guarantees recoverability. Journaling systems can provide – same durability semantics as FFS if log is forced to disk after each meta-data operation – the laxer semantics of Soft Updates if log writes are buffered until entire buffers are full Will discuss two implementations – LFFS-file – LFFS-wafs 19

LFFS-file Maintains a circular log in a pre-allocated file in the FFS (about 1% of file system size) Buffer header of each modified block in cache identifies the first and last log entries describing an update to the block LFFS-file maintains its log asynchronously – Maintains file system integrity, but does not guarantee durability of updates 20

LFFS-wafs (1/2) Implements its log in an auxiliary file system: Write Ahead File System (WAFS) – Can be mounted and unmounted – Can append data – Can return data by sequential or keyed reads Same checkpointing scheme and write-ahead logging protocol as LFFS-file 21

LFFS-wafs (2/2) Major advantage of WAFS is additional flexibility: – Can put WAFS on separate disk drive to avoid I/O contention – Can even put it in NVRAM LFS-wafs normally uses synchronous writes – Metadata operations are persistent upon return from the system call – Same durability semantics as FFS 22

23 Properties of Metadata Operations Feature Comparison Experimental Setup & Measurements

Properties of Metadata Operations Integrity – The file system is always recoverable Durability – Updates are persistent once the call returns Atomicity – No partial metadata operations are visible after recovery 24

Feature Comparison 25

Experimental Setup Software – Modified FreeBSD kernel and 2 journaling file system implementations (LFFS-wafs, LFFS-file) Hardware – 500 MHz Xeon Pentium III – 512 MB RAM – 3 x 9GB 10,000 RPM Seagate Cheetahs Compared performances of – Standard FFS – FFS mounted with the async option – FFS mounted with Soft Updates – FFS augmented with a file log using asynchronous log writes – FFS augmented with a WAFS log using Synchronous /asynchronous log writes WAFS log on same/different drive 26

Microbenchmark 27

Comments on Microbenchmark Results FFS-async performs best Original FFS, LFFS-wafs-2sync and LFFS-wafs-1sync perform worst – Synchronous log updates are costly LFFS-file outperforms LFFS-wafs-2async and LFFS-wafs- 1async – LFFS-file uses bigger block clusters for log writes Additional Results – Read/write performance identical for all systems – All async systems have similar create throughput – Soft updates has great delete performance due to its ability to do background work 28

Macrobenchmarks SSH-build – Unpacks, configures and builds ssh NetNews – Simulates the work of a news server SDET – Emulates user interactive software development workload Postmark – Designed to model the workload seen by ISPs under heavy load: combination of , news and e- commerce transactions 29

SSH Benchmark 30

NetNews Benchmark 31

Conclusions Journaling alone is not sufficient to “solve” the meta-data update problem – Cannot realize its full potential when synchronous semantics are required When that condition is relaxed, journaling and Soft Updates perform comparably in most cases 32

Problems Only a single file system with a single write-ordering model and a single approach to writing the journal was evaluated. Under what circumstances will soft update perform better and where will Journaling perform better were not close ended. It looks more of survey paper than conclusive stance over issues at hands. High performance applicants will find it too naive to accept as practioners guide (Who will be its first implementer). Work load taken were too sparse and were too high level to close end a discussion in particular discipline and does not offer a final say. Survey was good, but analysis lacked to comprehend a pin pointed viewpoint. Performance over array (disks) must have appealed high performance applicants, but paper does not provide any knowledge for such meaningful debate. 33