Robust Mounts

[Dikran Kassabian]

In article <1376 at smurf.ira.uka.de> urlichs at smurf.ira.uka.de writes:
>In comp.unix.questions deke at ee.rochester.edu writes:
>< 
>< In article <10284 at zodiac.ADS.COM> mliverig at spark.uucp writes:
>< >A few months back there was a discussion about making "nfs" networks
>< >robust. It was my impression that there was a consensus that:
>< >
>< >1) Soft mounts of read-write file systems would increase the risk of
>< >corrupting the file systems.
>< 
>How?

In the same way a system crash can result in corruption of local disks.
Summary information can be inaccurate depending on exactly when the crash
takes place, as it relates to pending disk writes.

An NFS rw,hard mount is a win in this case...  the process on the NFS client
hangs until the NFS mount becomes available again, and so gets to continue.
Not that this guarentees you a clean file-system, but I believe that your
chances are lots better.

>After all, whoever mounts the NFS volume is not doing any device-level stuff
>(it's Network _File_, not _Volume_ System, after all ;-) ), so how could a
>file system get corrupted?
>
>If you mean that the internal structure of some files could get screwed up, on
>the other hand, you're perfectly correct.

Absolutely;  I have seen this happen.

>< I agree with this, and stick to it as a loose rule.  There are some occasions
>< when I ignore it, after thinking it over carefully.
>< 
>Such as? (Inquiring minds want to know...)

Any time that small, infrequent writes from an NFS client are wanted, I'd
consider it.   Here's an example of when I'd use rw,soft:

* Server A has an area of disk I'd infrequently like to write to from client B.
* The data written will consist of small files which don't grow (perhaps from
	maintaining single record, statistical summaries).
* Server A has a tendancy of going down more often than I like, and I don't
	want to hang client B when this happens ... I'd rather restore the
	data files from backups and/or run fsck in the rare case when I 
	happened to be writing just as A crashed.

BUT:

My preferred solution would be to use SunOS automount(8) or Jan-Simon Pendry's
'amd'.  I'm still hoping someone will comment on my question, which
asked about automounter, and why it might be considered 'not yet safe'.

>
>-- 
>Matthias Urlichs

      ^Deke Kassabian,   deke at ee.rochester.edu   or   ur-valhalla!deke
   Univ of Rochester, Dept of EE, Rochester, NY 14627     (+1 716-275-3106)