Solving the ext3 latency problem [LWN.net]

Solving the ext3 latency problem

Posted Apr 18, 2009 16:14 UTC (Sat) by sbergman27 (guest, #10767)
In reply to: Solving the ext3 latency problem by chad.netzer
Parent article: Solving the ext3 latency problem

From Ted, in a relatively recent interview, speaking on *ext4* performance tips:

"If you dont need the security guarantees of what happens after a crash that are provided by data=ordered, try using the data=writeback mount option."

http://www.linux-mag.com/id/7272/2/

to post comments

Solving the ext3 latency problem

Posted Apr 18, 2009 23:22 UTC (Sat) by bojan (subscriber, #14302) [Link] (2 responses)

Compare that to this comment:

Fundamentally, the problem is caused by data=ordered mode. This problem can be avoided by mounting the filesystem using data=writeback or by using a filesystem that supports delayed allocation such as ext4. This is because if you have a small sqllite database which you are fsync(), and in another process you are writing a large 2 megabyte file, the 2 megabyte file wont be be allocated right away, and so the fsync operation will not force the dirty blocks of that 2 megabyte file to disk; since the blocks havent been allocated yet, there is no security issue to worry about with the previous contents of newly allocated blocks if the system were to crash at that point.

Contradictory, isn't it?

http://thunk.org/tytso/blog/2009/03/12/delayed-allocation-and-the-zero-length-file-problem/

Solving the ext3 latency problem

Posted Apr 19, 2009 0:59 UTC (Sun) by sbergman27 (guest, #10767) [Link]

I guess.

On a related note, if he thinks that writeback is good enough for ext3 because, after all, nobody runs Linux with multiple users... then is writeback also destined to be the default for ext4? Or is the idea to destabilize the thus far rock solid ext3 enough to make ext4 look better by comparison?

Solving the ext3 latency problem

Posted Apr 19, 2009 2:56 UTC (Sun) by sitaram (guest, #5959) [Link]

Actually, maybe not. This is not about security; as far as security goes ext4 is the same. What he is saying (or what I understand him to be saying) is that in ext4, due to delayed allocation, the performance issue with data=ordered is alleviated.

So I'd see this as "delayed allocation makes ordered almost as efficient as writeback", not "...makes writeback as secure as ordered"