Re: SSD + RAID

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.

--
Greg Smith    2ndQuadrant   Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com  www.2ndQuadrant.com