On 1/30/11 5:07 PM -0600 Stan Hoeppner wrote:
To be clear, for any subscribers who haven't followed all of the various filesystem and data security threads, with any modern *nix system, you WILL lose data when power fails. How much depends on how many writes to disk were in flight when the power failed, and how one has their RAID controller and inside-the-disk caches configured, whether using barriers, etc.
That's incorrect. When you fsync() a file, all sane modern filesystems guarantee no data loss, unless you tune that out administratively for performance reasons. If you use a log structured filesystem (like zfs or WAFL) you can optimize the performance as well. With other types of filesystems (like xfs), performance suffers severely under heavy sync write loads. As a reference point, ext3 with default settings guarantees data loss under normal conditions so I do not consider it a sane filesystem. You can tune that behavior out (so that you preserve data), but in that case ext3 operates with sub-par performance.
I believe I mentioned this when discussing the merits of XFS and ZFS with Frank, who stated Solaris/ZFS were immune to this, to which I called BS. They aren't immune, as Ted T'so clearly states. For those who don't know, Ted T'so is an MIT PH.D., is the creator of EXT2/3, and is to this day an active Linux kernel hacker/developer on filesystems and storage drivers.
Ted is a close acquaintance of mine, and if he indeed says what you said he says, he is wrong. More likely, he was simplifying or talking about certain cases, not the general case. There are two ways to guarantee no data loss with zfs, one is to disable the ZIL (low performance) and the 2nd is to use a slog (high performance). -frank
