Slower performance with ext4

[Chan Chung Hang Christopher]

>> You might want to also do a boot with your ext4 filesystems mounted with 
>> 'barrier=0' in fstab.
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> Can you explain what this does?
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It disables write barriers. Write barriers are enabled by default on 
ext4. Blast Mark, making me bone up on what is going on lately. :-)

That means that if write-caches are enabled on disks, you are at risk of 
losing data in the event of a sudden power loss but you get better 
performance in return. Write barriers allow you to have write-caches 
enabled and not have to risk losing data by ensuring that data is safely 
on disk before saying "It's done."

However, not everything disk related supports write-barriers, namely 
device-mapper, so if you use LVM or any md module other than raid1, you 
better turn write-caches off or get yourself a hardware raid card with 
bbu cache or a bbu nvram card and data=journal.