RELENG_5, snapshots and disk lock time

[Dave Knight]

 >On Mon, Mar 07, 2005 at 11:58:02AM -0500, Paul Mather wrote:
 >>On Mon, 2005-03-07 at 15:21 +0300, Dmitry Morozovsky wrote:
 >>> Dear colleagues,
 >>>
 >>> dumping the snapshot of 140G ufs2 fyle system under contemporary
 >>> RELENG_5 I found that during mksnap_ffs file system is 
unresponsible >>> even for reading for more than 3 minutes (it's on 
modern SATA disk
 >>> with 50+ MBps linear transfer).
 >>> Is it normal?
 >>
 >> Oddly enough, this happened to me last night on a RELENG_5 system. In
 >> my case, things were so bad that mksnap_ffs appeared to wedge
 >> everything, meaning I'll have to make a trek in to where the machine
 >> is located and press the ol' reset button to get things going again. 
 >> :-(

I am investigating using snapshots for backup purposes and am running 
into similar difficulties, on a 1TB FS it takes over an hour to create
a snapshot, during which time an errant ls or two can lock up the 
system. Reading through list archives suggests that the the amount of 
time it takes to create the snapshot is not something that is going to 
go away and that the issue of an ls in the .snap directory during 
snapshot creation lacks a fix and that best current practise is 'try to 
avoid that'.

 > Yes, this is normal.  See the documentation about the snapshots
 > implementation (a README in the kernel source tree, I think, and paper
 > written by Kirk).

That document also says:
"As is detailed in the operational information below, snapshots are 
definitely alpha-test code and are NOT yet ready for production use."
Is this the current opinion of snapshots ?

 >> The machine in question makes and mounts snapshots of all its
 >> filesystems for backup each night via Tivoli TSM.  This has worked
 >> flawlessly for many months.  Last night, I had many BitTorrent
 >> sessions active on the filesystem that wedged.  I guess the activity
 >> broke the snapshot mechanism. :-(  The odd thing is that it survived
 >> the night before, when there were also BitTorrent sessions active.
 >
 > It's possible there are still deadlock conditions in the snapshot
 > code.  Some familiarity with DDB would help to diagnose this (see the
 > chapter on kernel debugging in the developers' handbook).  You'd need
 > to work with Kirk to debug these, if you're willing.
 >
 >> I wonder how much activity mksnap_ffs can take?
 >
 > I don't think this is the issue, directly.
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