Re: common layout xattr

[Andreas Dilger <adilger@sun.com>]

On Jul 16, 2009  15:29 -0700, Sage Weil wrote:
> The fs would need to add in any unspecified local settings with defaults, 
> so that in general reading the xattr will still always fully describe the 
> layout.

The filesystem pretty much has to do that already, because currently no
layout information is passed to the filesystem with the majority of
file creates.

> So in your example, on fsX (Lustre) you might see
> 
>  chunk_bytes=65536
>  stripe_count=32
>  mirror_count=3
>  raid_type=1+0
> 
> On fsY (ceph) you might see
> 
>  chunk_bytes=65536
>  stripe_count=32
>  mirror_count=3            (ceph ignores)
>  raid_type=1+0             (ceph ignores)
>  max_object_size=64MB      (ceph adds)

Hmm, I thought Ceph had replication for files?  "mirror_count" was intended
to indicate the number of copies of that file that are maintained.  Maybe it
needs a better name?

> and back on fsX (Lustre) you'd get
> 
>  chunk_bytes=65536
>  stripe_count=32
>  mirror_count=3
>  raid_type=1+0
>  max_object_size=64MB     (lustre ignores)

Exactly what I was thinking.

> How would you indicate which parameters are being ignored?  Something 
> easily parsed (and ignored) when setting the xattr.

I was just thinking something like a prefix or postfix character:

chunk_bytes=65536
stripe_count=32
mirror_count=3
raid_type=1+0
*max_object_size=64MB

or

max_object_size=64MB*

It might also make sense to use "*" (or some other character) to
indicate values that are the filesystem-wide default values, and
not store them either, but that isn't a well-formed opinion yet.

Even dynamically-added "comments" might be OK, like:

chunk_bytes=65536	# default
stripe_count=32		# default
mirror_count=3
raid_type=1+0		# default
max_object_size=64MB	# unknown

> As long as the common parameter names are somewhat standardized, this 
> seems straightforward enough.

Good, I don't want anything too complex.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.