Tom Lane wrote:
[EMAIL PROTECTED] writes:IDE or scsi? If IDE: Write cache on or off? Which 2.4 kernel?
The improvements were REALLY astounding, and I would like to know if other
Linux users see this performance increase, I mean, it is almost 8~10 times
faster than using fsync.
Furthermore, it seems to also have the added benefit of reducing the I/O
storm at checkpoints over a system running with fsync off.
What size transactions are you using in your tests?
For a system with small transactions (not much more than 1 page worth of WAL traffic per transaction) I'd be pretty surprised if there was any real difference at all. There certainly should not be any difference in terms of the number of physical writes. We have seen some platforms where fsync() is inefficiently implemented and requires more kernel overhead than is reasonable --- not for I/O, but just to look through the kernel buffers and confirm that none of them need flushing. But I didn't think Linux was one of these.
The numbers are very high - it could be a side effect of write caching by the disks. I think some Suse 2.4 kernels have partial support for reliable fsync even if the write cache is on (i.e. fsync issues a cache flush command to the disk), but not all code paths are handled. Perhaps fsync is handled and O_SYNC is not handled.
I could try to find the details.
-- Manfred
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