System hanging during dump

[Peter Jeremy]

On 2008-Oct-15 02:08:48 -0700, Jeremy Chadwick <koitsu at FreeBSD.org> wrote:
>On Wed, Oct 15, 2008 at 07:58:43PM +1100, Peter Jeremy wrote:
>> On 2008-Oct-15 01:35:38 -0700, Jeremy Chadwick <koitsu at FreeBSD.org> wrote:
>> >On Wed, Oct 15, 2008 at 07:24:28PM +1100, Peter Jeremy wrote:
>> >> Last night, I attempted a full, compressed backup of my 181GB /home
>> >> (on a PATA disk) to a remote system.  The backup started at 2159 and
>> >> everything appeared normal until about 0040 when the system became
>> >> non-responsive and this lasted until the dump completed at 1033.  This
>> >> is the first full backup of /home I've made for several years (due to
>> >> lack of space).
>> ...
>> >It's a known problem documented in my Wiki -- see "dump/restore".  Note
>> >the part about UFS2 snapshot generation.  I'm almost certain this is
>> >what you're describing.
>> 
>> * UFS2 snapshot generation (mksnap_ffs, dump -L) takes too long; system is unusable during this time
>>   In my case, snapshot creation took ~4 minutes.  The system was
>>   running normally for 2.6 hours after snapshot creation completed
>>   before it froze.
>
>Did you read the References, including the one from myself?

Yes.  In my case, dump started and ran mksnap_ffs.  About 4 minutes
later, actual dumping started and data streaming continued for about
12.6 hours.  The system froze about 2.6 hours into the dump (after
dump had written about 31GB).

>Snapshot generation in some cases took only minutes, but *removal* of
>the generated the snapshot took 1.5 hours or more, hanging the system
>until the removal was complete.

Based on progress reports from both dump and my fifo process, the
snapshot removal began about 10 hours _after_ the system froze
(during this time, dump wrote about 143GB).  Given the timeline,
it's fairly clear that neither mksnap_ffs nor the 'rm snapshot'
were running at the time the system froze.  I am therefore quite
confident that the problem I saw is not related to either creation
or removal of snapshots.

I have been using FreeBSD snapshots for many years and am quite
familiar with their quirks.  I have never seen this particular
problem before.  (And FWIW, I _am_ using Doug Ambrisko's patch to
ffs_snapshot.c).

-- 
Peter Jeremy
Please excuse any delays as the result of my ISP's inability to implement
an MTA that is either RFC2821-compliant or matches their claimed behaviour.
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