[CentOS] Restart after crash

[Bryan J. Smith]

Les Mikesell <lesmikesell at gmail.com> wrote:
> Do you have some reason to think it is better to run a
> full fsck than to replay the journal?

Actually, unless you're doing full data journaling
(especially with a NVRAM board), it's almost always _safer_
to run a full fsck instead of just doing a journal replay.

> I thought that was the point of using a journalled
> filesystem.

The point of using a meta-data journaling filesystem is to
[near] guarantee the filesystem into a consistent state
without having to run a time-consuming fsck.  Journaling
doesn't guarantee anything as far as reliability (and can
actually be worse -- don't get me started on NTFS ;-).

Full data journaling filesystems are better (and worse in
some ways) than meta-data journaling filesystem.  But full
data journaling filesystems using a NVRAM for the journal is
near-perfect because you have a commit to near-instaneous,
battery-backed DRAM -- which is the journal (not disk) --
before any flush to disk.

(Tangent Warning=ON)

Tying in a recent thread, NetApp's Data OnTap OS is designed
to buffer to NVRAM as the journal for the WAFL filesystem. 
That's why it recovers so quickly and near perfectly -- the
OS/filesystem is designed around that advanced hardware
capability (which all NetApp filers have).

VALinux used to sell filers with a similar (albeit more
conventional) option using a PCI NVRAM board, Ext3 in
full-data journaling and NFS in sync mode (instead of async).
 It wasn't any faster than NFS async, but it was much, much
safer (basically async for free).


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