On 11/23/16 1:11 PM, Laszlo Ersek wrote:
Separately, a small note on ext4 (because you mention it above). I seem to
recall a filesystem expert colleague of mine advise *against* using journaled
filesystems for booting with e.g. grub2. The argument goes (if I recall right),
XFS is considered to be in clean state if the data has made it to the final
location *or* the persistent journal. When you cleanly unmount (or remount r/o)
and shut down, the journal will be flushed to the final location, so a boot
loader that doesn't know about the journal will read consistent data. However,
if you crash *without* data loss, then part of the data might be in the journal
only, and only clients that can read the journal will see consistent data.
This might or might not be
an issue, depending on what the point of the exercise is.
The problem of course is that "reset" (reboot), "reset -s" (shutdown)
etc. don't have any hooks a driver can use to flush data, so there will
sometimes (depending on how long ago data was written) be an unclean
unmount. At least when booting an OS there's an ExitBootServices event
available to make sure any data is flushed before boot services ends.
Though I've seen one person talking about how there at least used to be
a bug that means ExitBootServices isn't/wasn't called for one OS.
--
Rebecca
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