Re: Why is fsync so much slower than O_SYNC?

[Jeffrey Siegal]

Andrew Morton wrote:
> This is because ext3 is being dumb.  It is forcing a journal commit even
> when the inode has not been altered.
>
> Here's a moderately-tested 2.6 fix.  A 2.4 fix would be similar.
>
>
> File overwrite:
>
> 	vmm:/mnt/hda5> ~/fsync-is-sucky 
> 	O_SYNC:
> 	Creating
> 	Starting
> 	iter = 1000, latency = 0.191464ms
>
> 	O_DSYNC:
> 	Creating
> 	Starting
> 	iter = 1000, latency = 0.181637ms
>
> 	fsync:
> 	Creating
> 	Starting
> 	iter = 1000, latency = 0.186326ms
>
> 	fdatasync:
> 	Creating
> 	Starting
> 	iter = 1000, latency = 0.193013ms

Hmm, this doesn't seem right.  For an overwrite, it should still be 
waiting for just the data page to be written out, and that should take 
about 8ms (for a 7200rpm drive), and the append case should take longer, 
since it requires writing both the data page and then the journal.

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