Freeze IO

  • Comments posted to this topic are about the item Freeze IO

  • Good question on an undocumented(?) command, thanks.

    ...

  • Good to know!

    😀

  • use at your peril I guess?

    Are these the commands used by hardware based backup solutions?

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  • Never knew of it. Thanks, Steve for sharing.

  • When was this introduced? We're on 2008R2. Is there "documentation" somewhere of the undocumented DBCC commands for 2008R2?

  • My first question of the day, yahoo.

  • HappyGeek (5/24/2016)


    Good question on an undocumented(?) command, thanks.

    Yes, undocumented

  • george sibbald (5/25/2016)


    use at your peril I guess?

    Are these the commands used by hardware based backup solutions?

    don't think so. Those use the VDI interface, which doesn't freeze IO, but rather allows the product capture log records for changes.

  • George Vobr (5/25/2016)


    Never knew of it. Thanks, Steve for sharing.

    Me, either. It was a surprise to read about it and try it.

  • Steve Jones - SSC Editor (5/25/2016)


    george sibbald (5/25/2016)


    use at your peril I guess?

    Are these the commands used by hardware based backup solutions?

    don't think so. Those use the VDI interface, which doesn't freeze IO, but rather allows the product capture log records for changes.

    Looking at the server logs, we see messages identical to those created when SnapManager does its thing. Well, mostly. The Thaw message is the same but, for some reason, I'm not seeing anything in my playpit's server log for a Freeze. Hmm.

    Thomas Rushton
    blog: https://thelonedba.wordpress.com

  • Ah, snaps are different. That's not a backup (usually), but a copy of the disk at a point in time. It should use the freeze to mark a stamp (somehow) that lets them copy blocks on write. They otherwise just copy blocks, but writes get priority for duplication.

  • Interesting...

  • Nice Question.

    1 question, what is the suitable situation to use such command ? ( yes, not recommended to use in Production, I know ... but eager to know how we can get benefit out of it...so that can try atleast in my local sql instance. 🙂 🙂 )

    Thanks.

  • Never heard of this command before, didn't find any documentation on it, so I created a database to try it one (not wanting to risk doing something horrid to any of my existing databases, not even to my playpen database); so discovered it suspended writes, and apparently did nothing else. Then came the fun bit: trying to guess the command to reverse it, then seeing if I could drop the database without first reversing it (no, that counts as a write so it gets suspended), then seeing if I could stop the service without first reversing it (no, that too seems to get suspended). So killed off the suspended sessions (one containing the initial write that got suspended, another containing the drop database that got suspended) and tried dropping it again - still hung. So answered the question (correctly, of course) and followed up the reference in the explanation and found the command to reverse it there. "thaw" instead of "unfreeze", seemed obvious once I'd seen it but I'd tried half a dozen incorrect guesses and never thought of "thaw". And thaw_io thawed it alright, so the stop service commands were no longer suspended and the service stopped.

    So I was rather careless, trying a command when I neither knew what it actually did nor had any idea how to reverse any effect it might have on my system. But that's one of the reasons I have backups (and not just for databases), it allows me to be a bit careless when I feel like learning by playing with the system to see what happens (I don't play on anything like a production system, of course). It would have been less fun to just do a google search on the whole web (and get the info on the command and on how to undo it) instead of experimenting when there was no documentation on the command in the MSDN library.

    Tom

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