zfs discuss - Jul 2011 - What is ZFS metadata in regard to caching and redundant storage policy?

I just found that I do not exactly know: What is ZFS metadata in regard
to caching and redundant storage policy? What sort of blocks does it
include and what - doesn''t?

I know that there is some metadata which can be prioritized in ARC
with "primarycache" (or in L2ARC with "secondarycache")
attributes.
There is some metadata which is reportedly stored in a mirrored way
regardless of a raidzN layout used for data blocks, and this matches
what I see in iostat output.

But which data is it - only ZFS infrastructure (indirect blocks et al), or
also the filesystem directory tree?

The latter I am interested in particular: if I set caching to metadata,
should I expect recursive directory walks to be crippled or boosted? :)

Thanks,
//Jim

Hi Jim, 

Non-metadata is the level zero blocks (leaf blocks) of those zfs objects 
that store user payload. (plain files, volumes and the like) 

Everything else is metadata. 

The place to look is dmu.c. 

Any object type enumerated in dmu_ot that is flagged "TRUE" is
metadata
in its entirety, including its payload ( ie its level zero blocks). 

Example: 
const dmu_object_type_info_t dmu_ot [ DMU_OT_NUMTYPES ] = { {
byteswap_uint8_array , TRUE , "unallocated" }, { zap_byteswap , TRUE ,
"object directory" }, { zfs_oldacl_byteswap , TRUE , "ZFS V0
ACL" },
.... 

{ byteswap_uint8_array , FALSE , "ZFS plain file" }, { zap_byteswap ,
TRUE , "ZFS directory" }, { zap_byteswap , TRUE , "ZFS master
node" },
{ zap_byteswap , TRUE , "ZFS delete queue" }, 
{ byteswap_uint8_array , FALSE , "zvol object" }, 
Cheers, 

Steve Gonczi 

----- Original Message -----
> I just found that I do not exactly know: What is ZFS metadata in regard 
> to caching and redundant storage policy? What sort of blocks does it 
> include and what - doesn''t? 
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