Vault 2015 Notes: Second Day Afternoon

Afternoon session in Ted Ts’o’s talk on lazytime mount option.  On surface, his talk shared many with the paper I wrote years back. He emphasized on tail latency than average (and I agree).  After spending time on showing where latency irregularity coming from, he pointed out that mtime update was problematic. Not flushing mtime was scary but Ted said other information (i_size, etc) could help you find if files were modified. And if no i_size change (like database), application usually didn’t care about mtime. He added dirty flags to inode as hint for fdata_sync (no mtime change) or fsync (mtime change). ext4 is the current new lazytime compliant filesystem. He ended with a ftrace demo. His multithreaded random write fio benchmark on RAM Disk showed double bandwidth, lockstat showed locking contention on journal went away. He also mentioned of DIO read lock removal after eliminating the chance of reading stale data on the write path. That dioread_nolock (?) option enabled ext4 to DIO read in parallel at raw high speed Flash speed.

Next topic was loaded with all buzz: Multipath, PCI-e, NVM. He showed a chart pointing to software was the last one to reduce latency in NVM era.

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