dangers of delaying an fsck on busy fileserver ?

[Gore Jarold]

--- Scott Long <scottl at samsco.org> wrote:

> Gore Jarold wrote:
> > --- Scott Long <scottl at samsco.org> wrote:
> > 
> > 
> >> In an ideal world, the only consequence of
> delaying
> >> bgfsck is that
> >> not all filesystem blocks will be marked free
> that
> >> should be.  So
> >> if you deleted a large tree of files before the
> >> crash, those blocks
> >> might still show up in use until bgfsck
> completes.
> > 
> > 
> > Thank you.  Would _you_ do this with valuable data
> ?
> > 
> 
> Very good question =-)  If you're using softupdates
> then any
> damage will have been done when the hard shutdown
> happens; bgfsck
> won't create any new damage.  The biggest problem of
> bgfsck beyond
> the i/o slowness and near deadlocks that it can
> create (modulo the
> fixes that the Kostik is working on) is that if it
> does encounter
> damage that it can't fix automatically, it exits and
> leaves the 
> filesystem inconsistent.  So you need to keep a very
> close eye on
> your logs and check for this, then schedule downtime
> when it happens
> so you can babysit a full fsck.


Ahhh... I think you may have misunderstood my original
question.  What I am saying is, I don't _ever_ want to
do a background fsck.  My systems are too busy (and
have too large of disks) to deal with the (current)
baggage of making a 4 TB snapshot and then
bg_fsck'ing.

What I am saying is the following:

- I set background_fsck_delay="86400"

- I tell datacenter techs NOT to call me when the
system crashes - just to hit reset.

- users bang on the system, as normal, for X hours -
all the while the filesystems are _dirty_ and nothing
is being done about it

- I wake up hours later, unmount the filesystems, and
foreground fsck them

My goal in all of this is to keep from being woken up
in the middle of the night.  I don't care about the
downtime to the system when I eventually do foreground
fsck them, I just don't want to do it in the middle of
the night _and_ I don't want my users to have to sit
around waiting for me to do the fsck _on top of_ the
fsck downtime itself.

So ... comments ?  I _suspect_ the conclusions are
about the same - running on a dirty FS is the same as
running on a dirty FS while being bg_fsck'd ... but I
want to make sure...


       
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