Re: [HPDD-discuss] Regarding the Extended Attributes structure in MDS
On Sep 5, 2015, at 14:37, Akhilesh Gadde <akhilesh.gadde(a)stonybrook.edu> wrote:
I have a query regarding the file metadata stored in MDS/MDT.
I am trying to understand what information would be normally present in Inode EA
attributes for a directory (not file). I tried searching for it but could not find much
information.
- When a new file is created, MDS would create a new FID with EA attributes. File EA
given to client would contain objects on which the data would be written and their
corresponding OSTs.
This is the "lov" xattr that holds the file layout. The Lustre FID is also
stored in the "lma" xattr on the inode (both file and directory inodes).
Lustre also stores the "link" xattr the holds the filename and parent FID for
each link to the file. That allows generating a pathname from the FID for each file.
(This ensures separation of metadata from actual data since now the
client can write directly to OSTs without contacting MDS)
Correct.
- I would like to know what happens similarly when a new directory is
created. Does MDS also allocate objects and their corresponding OSTs (considering the
directory data is nothing but it's containing filenames and their inode numbers)
No, the directory is held only on the MDT. It also has an "lma" xattr to hold
the FID. In Lustre 2.8 and later a single directory may be striped across multiple MDTs,
analogous to how a file is striped over multiple OSTs, in which case it will have an
"lov" xattr to hold the directory striping layout.
Could you please help me in clarifying the above query and also
correct me if I'm misinterpreting some information.
Regards,
Akhilesh.
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