Since le modications may not b e written back to the

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Unformatted text preview: es. AFS, Coda (in its strongly-connected mode), and NFS write file modifications synchronously to the server on close. Directory caching in these file systems is writethrough. Sp eculator would substantially improve write p er19 formance in these file systems by hiding the latency of these synchronous op erations. Since file modifications may not b e written back to the server until the file is closed, data can b e lost in the event of a system crash. Echo caches modifications for longer p eriods of time and writes back mod- • If no conflicting writes, longer Tx better • But possibly harder for app to rollback • What about nontransactional apps? 18 200 NFS SpecNFS BlueFS 10 100 From Nightingale, SOSP 2005 5 0 0 No delay 30ms delay This figure shows the time to run the PostMark benchmark. Each value is the mean of 5 trials—the error bars are 90% confidence intervals. Note that the scale of the y-axis differs 20 How? Client 1 Modify A Modify B Server Client 1 Client 2 Modify A Write speculate Commit Server • create_speculation Client 2 Write+Commit – Normal client code goes here Modify B Getattr Write Open C Open B speculate Commit Getattr Getattr Getattr Open C Open B speculate Open B (a) Unmodified NFS (b) Sp eculative NFS Figure 1: Example ofNightingale, SOSP 2005 cution for NFS From speculative exe 21 processes sp...
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