Wandering Thoughts archives

2018-03-19

Some exciting ZFS features that are in OmniOS CE's (near) future

I recently wrote about how much better ZFS pool recovery is coming, which reported on Pavel Zakharov's Turbocharging ZFS Data Recovery. In that, Zakharov said that the first OS to get it would likely be OmniOS CE, although he didn't have a timeline. Since I just did some research on this, let's run down some exciting ZFS features that are almost certainly in OmniOS CE's near future, and where they are.

There are two big ZFS features from Delphix that have recently landed in the main Illumos tree, and a third somewhat smaller one:

  • This better ZFS pool recovery, which landed as a series of changes culminating in issue 9075 in February or so. Although I can't be sure, I believe that a recovered pool is fully compatible with older ZFS versions, although for major damage you're going to be copying data out of the pool to a new one.

  • The long awaited feature of shrinking ZFS pools by removing vdevs, which landed as issue 7614 in January. Using this will add a permanent feature flag to your pool that makes it fully incompatible with older ZFS versions.

  • A feature for checkpointing the overall ZFS pool state before you do potentially dangerous operations and can then rewind to them, issue 9166, which landed just a few days ago. Since one of the purposes of better ZFS pool recovery is to provide (better) recovery over pool configuration changes, I suspect that this new pool checkpointing helps out with it. This makes your pool relatively incompatible with older ZFS versions while a checkpoint exist.

(Apparently Delphix was only able to push these upstream from their own code base to Illumos and OpenZFS relatively recently.)

All of these features have been pulled into the 'master' branch of the OmniOS CE repository from the main Illumos repo where they landed. Unless something unusual happens, I would expect them all to be included in the next release of OmniOS CE, which their release schedule says is to be r151026, expected some time this May. This will not be an LTS release; if you want to wait for an LTS release to have these features, you're waiting until next year. Given the likely magnitude of these changes and the relatively near future release of r151026, I wouldn't expect OmniOS CE to include these in a future update to the current r151024 or especially r151022 LTS.

Since OmniOS CE Bloody integrates kernel and user updates on a regular basis, I suspect that it already has many of these features and will pick up the most recent one very soon. If this is so, it gives OmniOS people a clear path if they need to recover a damaged pool; you can boot a Bloody install media or otherwise temporarily run it, repair or import the pool, possibly copying the data to another pool, and then probably revert back to running your current OmniOS with the recovered pool.

Sidebar: How to determine this sort of stuff

The most convenient way is to look at the the git log for commits that involve, say, usr/src/uts/common/fs/zfs, the kernel ZFS code, in the OmniOS CE repo. In the Github interface, this is drilling down to that directory and then picking the 'History' option; a convenient link for this for the 'master' branch is here.

Each OmniOS CE release gets its own branch in the repo, named in the obvious way, and each branch thus has its own commit history for ZFS. Here is the version for r151024. Usefully, Github forms the URLs for these things in a very predictable way, making it very easy to hand-write your own URLs for specific things (eg, the same for r151022 LTS, which shows only a few recent ZFS changes).

There's no branch for OmniOS CE Bloody, so I believe that it's simply built from the 'master' branch. It is the bleeding edge version, after all.

ZFSOmniosCEComingChanges written at 01:24:50; Add Comment

2018-03-17

Much better ZFS pool recovery is coming (in open source ZFS)

One of the long standing issues in ZFS has been that while it's usually very resilient, it can also be very fragile if the wrong things get damaged. Classically, ZFS has had two modes of operation; either it would repair any damage or it would completely explode. There was no middle ground of error recovery, and this isn't a great experience; as I wrote once, panicing the system is not an error recovery strategy. In early versions of ZFS there was no recovery at all (you restored from backups); in later versions, ZFS added a feature where you could attempt to recover from damaged metadata by rewinding time, which was better than nothing but not a complete fix.

The good news is that that's going to change, and probably not too long from now. What you want to read about this is Turbocharging ZFS Data Recovery, by Pavel Zakharov of Delphix, which covers a bunch of work that he's done to make ZFS more resilient and more capable of importing various sorts of damaged pools. Of particular interest is the ability to likely recover at least some data from a pool that's lost an entire vdev. You can't get everything back, obviously, but ZFS metadata is usually replicated on multiple vdevs so losing a single vdev will hopefully leave you with enough left to at least get the rest of the data out of the pool.

(I saw this article via, itself via a retweet by @richardelling.)

All of this is really great news. ZFS has long needed better options for recovery from various pool problems, as well as better diagnostics for failed pool imports, and I'm quite happy that the situation is finally going to be improving.

The article is also interesting for its discussion of the current low level issues involved in pool importing. For example, until I read it I had no idea about how potentially dangerous a ZFS pool vdev change was due to how pool configurations are handled during the import process. I'd love to read more details on how pool importing really works and what the issues are (it's a long standing interest of mine), but sadly I suspect that no one with that depth of ZFS expertise has the kind of time it would take to write such an article.

As far as the timing of these features being available in your ZFS-using OS of choice goes, his article says this:

As of March 2018, it has landed on OpenZFS and Illumos but not yet on FreeBSD and Linux, where I’d expect it to be upstreamed in the next few months. The first OS that will get this feature will probably be OmniOS Community Edition, although I do not have an exact timeline.

If you have a sufficiently important damaged pool, under some circumstances it may be good enough if there is some OS, any OS, that can bring up the pool to recover the data in it. For all that I've had my issues with OmniOS's hardware support, OmniOS CE does have fairly decent hardware support and you can probably get it going on most modern hardware in an emergency.

(And if OmniOS can't talk directly to your disk hardware, there's always iSCSI, as we can testify. There's probably also other options for remote disk access that OmniOS ZFS and zdb can deal with.)

PS: If you're considering doing this in the future and your normal OS is something other than Illumos, you might want to pay attention to the ZFS feature flags you allow to be set on your pool, since this won't necessarily work if your pool uses features that OmniOS CE doesn't (yet) support. This is probably not going to be an issue for FreeBSD but might be an issue for ZFS on Linux. You probably want to compare the ZoL manpage on ZFS pool features with the Illumos version or even the OmniOS CE version.

Sidebar: Current ZFS pool feature differences

The latest Illumos tree has three new ZFS pool features from Delphix: device_removal, obsolete_counts (which enhances device removal), and zpool_checkpoint. These are all fairly recent additions; they appear to have landed in the Illumos tree this January and just recently, although the commits that implement them are dated from 2016.

ZFS on Linux has four new pool features: large_dnode, project_quota, userobj_accounting, and encryption. Both large dnodes and encryption have to be turned on explicitly, and the other two are read-only compatible, so in theory OmniOS can bring a pool up read-only even with them enabled (and you're going to want to have the pool read-only anyway).

ZFSPoolRecoveryComing written at 00:54:35; Add Comment


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