Thanks again Eric. Lots of food for thought here, so much head scratching I'll
get dandruff... Mark
On Wednesday 25 July 2007 20:30, Eric Schrock (Eric Schrock
<eric.schrock at sun.com>) may have written:
> On Wed, Jul 25, 2007 at 08:19:07PM +0200, Mark Furner wrote:
> > Many thanks for the quick reply, Eric. ZFS is quite awesome, but also
> > awesomely complicated.
> >
> > What did you mean with
> >
> > >For *filesystems*, you can consult the per-pool history.
> >
> > Is this the vdev label uberblock / MOS thing?
>
> Yes, the history is stored in the MOS.
>
> > > It is part of the MOS (meta object set), which is the root of all
> > > pool-wide metadata.
> >
> > I'm still trying to work out where the MOS can be found on disk. The
> > uberblock points to it, but is it outside the vdev label? The diagram on
> > p. 32 of the "ZFS On-Disk Specification" is a bit ambiguous here. Where
> > does the file-system metadata end and the files / file metadata /
> > directories begin?
>
> The uberblock points the MOS:
>
> struct uberblock {
> uint64_t ub_magic; /* UBERBLOCK_MAGIC */
> uint64_t ub_version; /* SPA_VERSION */
> uint64_t ub_txg; /* txg of last sync */
> uint64_t ub_guid_sum; /* sum of all vdev guids */
> uint64_t ub_timestamp; /* UTC time of last sync */
> blkptr_t ub_rootbp; /* MOS objset_phys_t */
> };
>
> The 'ub_rootbp' points to the MOS, from which all pool data and metadata
> can be discovered. It is a normal block pointer, which means it can be
> stored on any of the vdevs (several of them, thanks to ditto blocks).
> Only the vdev label (including the uberblock) has a fixed location,
> everything is derived from that.
>
> For what's contained in the MOS, check out the definition for
> DMU_POOL_DIRECTORY_OBJECT:
>
> /*
> * The names of zap entries in the DIRECTORY_OBJECT of the MOS.
> */
> #define DMU_POOL_DIRECTORY_OBJECT 1
> #define DMU_POOL_CONFIG "config"
> #define DMU_POOL_ROOT_DATASET "root_dataset"
> #define DMU_POOL_SYNC_BPLIST "sync_bplist"
> #define DMU_POOL_ERRLOG_SCRUB "errlog_scrub"
> #define DMU_POOL_ERRLOG_LAST "errlog_last"
> #define DMU_POOL_SPARES "spares"
> #define DMU_POOL_DEFLATE "deflate"
> #define DMU_POOL_HISTORY "history"
> #define DMU_POOL_PROPS "pool_props"
>
> Most of this is pool-wide metadata, but the 'root_dataset' is the link
> into the DSL (dataset and snapshot layer) which has a list of all the
> filesystems (among other things). Each filesystem has its own data
> structure that is the root of the actual filesystem data.
>
> Hope that helps,
>
> - Eric
>
> --
> Eric Schrock, Solaris Kernel Development
> http://blogs.sun.com/eschrock
