On 04/22/2011 09:32 AM, Simon Riggs wrote:
OK, that's good, but ISTM we still have a hole during RemoveOldXlogFiles() where we don't fsync or open/close the file, just rename it.
This is also something that many applications rely upon working as hoped for here, even though it's not technically part of POSIX. Early versions of ext4 broke that, and it caused a giant outcry of complaints. http://www.h-online.com/open/news/item/Ext4-data-loss-explanations-and-workarounds-740671.html has a good summary. This was broken on ext4 from around 2.6.28 to 2.6.30, but the fix for it was so demanded that it's even been ported by the relatively lazy distributions to their 2.6.28/2.6.29 kernels.
There may be a small window for metadata issues here if you've put the WAL on ext2 and there's a crash in the middle of rename. That factors into why any suggestions I make about using ext2 come with a load of warnings about the risk of not journaling. It's hard to predict every type of issue that fsck might force you to come to terms with after a crash on ext2, and if there was a problem with this path I'd expect it to show up as something to be reconciled then.
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