Greg Smith wrote: > Scott Carey wrote: >> For your database DATA disks, leaving the write cache on is 100% >> acceptable, >> even with power loss, and without a RAID controller. And even in >> high write >> environments. >> >> That is what the XLOG is for, isn't it? That is where this behavior is >> critical. But that has completely different performance requirements >> and >> need not bee on the same volume, array, or drive. >> > At checkpoint time, writes to the main data files are done that are > followed by fsync calls to make sure those blocks have been written to > disk. Those writes have exactly the same consistency requirements as > the more frequent pg_xlog writes. If the drive ACKs the write, but > it's not on physical disk yet, it's possible for the checkpoint to > finish and the underlying pg_xlog segments needed to recover from a > crash at that point to be deleted. The end of the checkpoint can wipe > out many WAL segments, presuming they're not needed anymore because > the data blocks they were intended to fix during recovery are now > guaranteed to be on disk. Guys, read that again. IF THE DISK OR DRIVER ACK'S A FSYNC CALL THE WAL ENTRY IS LIKELY GONE, AND YOU ARE SCREWED IF THE DATA IS NOT REALLY ON THE DISK. -- Karl
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