zfs discuss - Oct 2007 - safe zfs-level snapshots with a UFS-on-ZVOL filesystem?

I had some trouble installing a zone on ZFS with S10u4
(bug in the postgres packages) that went away when I  used a
ZVOL-backed UFS filesystem
for the zonepath.

I thought I''d push on with the experiment (in the hope Live Upgrade
would be able to upgrade such a zone).
It''s a bit unwieldy, but everything worked reasonably well -
performance isn''t much worse than straight ZFS (it gets much faster
with compression enabled, but that''s another story).

The only fly in the ointment is that ZVOL level snapshots don''t
capture unsynced data up at the FS level. There''s a workaround at:

  http://blogs.sun.com/pgdh/entry/taking_ufs_new_places_safely

but I wondered if there was anything else that could be done to avoid
having to take such measures?
I don''t want to stop writes to get a snap, and I''d really like
to avoid UFS
snapshots if at all possible.

I tried mounting forcedirectio in the (mistaken) belief that this
would bypass the UFS
buffer cache, but it didn''t help.

-- 
Rasputin :: Jack of All Trades - Master of Nuns
http://number9.hellooperator.net/

Dick Davies wrote:
> I had some trouble installing a zone on ZFS with S10u4
> (bug in the postgres packages) that went away when I  used a
> ZVOL-backed UFS filesystem
> for the zonepath.
> 
Hi
Out of interest what was the bug.

Enda
> I thought I''d push on with the experiment (in the hope Live
Upgrade
> would be able to upgrade such a zone).
> It''s a bit unwieldy, but everything worked reasonably well -
> performance isn''t much worse than straight ZFS (it gets much
faster
> with compression enabled, but that''s another story).
> 
> The only fly in the ointment is that ZVOL level snapshots don''t
> capture unsynced data up at the FS level. There''s a workaround at:
> 
>   http://blogs.sun.com/pgdh/entry/taking_ufs_new_places_safely
> 
> but I wondered if there was anything else that could be done to avoid
> having to take such measures?
> I don''t want to stop writes to get a snap, and I''d really
like to avoid UFS
> snapshots if at all possible.
> 
> I tried mounting forcedirectio in the (mistaken) belief that this
> would bypass the UFS
> buffer cache, but it didn''t help.
>

On Mon, 8 Oct 2007, Dick Davies wrote:
> I had some trouble installing a zone on ZFS with S10u4
> (bug in the postgres packages) that went away when I  used a
> ZVOL-backed UFS filesystem
> for the zonepath.
>
> I thought I''d push on with the experiment (in the hope Live
Upgrade
> would be able to upgrade such a zone).
> It''s a bit unwieldy, but everything worked reasonably well -
> performance isn''t much worse than straight ZFS (it gets much
faster
> with compression enabled, but that''s another story).
>
> The only fly in the ointment is that ZVOL level snapshots don''t
> capture unsynced data up at the FS level. There''s a workaround at:
>
>  http://blogs.sun.com/pgdh/entry/taking_ufs_new_places_safely
>
> but I wondered if there was anything else that could be done to avoid
> having to take such measures?
> I don''t want to stop writes to get a snap, and I''d really
like to avoid UFS
> snapshots if at all possible.
Hmm - "Difficult Problem" (TM :), that is.
UFS, by design, isn''t atomic / self-consistent-at-all-times. Not even
with
UFS logging active, because the latter doesn''t actually log userdata.

Which is why establishing a snapshot on UFS, whatever method (and that 
includes UFS'' fssnap), involves creating a write barrier temporarily.
You
need to flush all data on UFS, while at the same time blocking updates 
beyond that point, before UFS (or, if not using fssnap, the admin) can 
''signal'' to the lower levels that a snapshot is safe to create
now.

One could conceive a (relatively small) codechange in UFS that would make 
this happen under the hood, e.g. in the form of an ioctl - "flush things 
and signal the ''driver level''
ioctl(SNAP_ENABLE_NOW_SAFE)". But that
would, although you would then not issue the "lockfs -w/u" yourself,
still
involve the temporary write barrier. Just as with fssnap, where this also 
happens under the hood.

Without userdata logging/journaling, it seems pretty difficult to get 
around that barrier constraint, though if you''d have suggestions as
"how
to do it in UFS", we should start talking about it. 
ufs-discuss at opensolaris.org is open :)

As said - "Difficult problem".
>
> I tried mounting forcedirectio in the (mistaken) belief that this
> would bypass the UFS
> buffer cache, but it didn''t help.
Nope, it''s a design issue in UFS; no userdata logging, no point-in-time
consistency (for "everything").

FrankH.
>
> -- 
> Rasputin :: Jack of All Trades - Master of Nuns
> http://number9.hellooperator.net/
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