[Gluster-devel] GFID to Path Conversion

Aravinda avishwan at redhat.com
Wed Dec 9 05:47:58 UTC 2015


Sharing draft design for GFID to Path Conversion.(Directory GFID to Path is
very easy in DHT v.1, this design may not work in case of DHT 2.0)

Performance and Storage space impact yet to be analyzed.

Storing the required informaton
Metadata information related to Parent GFID and Basename will reside
with the file. PGFID and hash of Basename will become part of Xattr
Key name and Basename will be saved as Value.

     Xattr Key = meta.<PGFID>.<HASH(BASENAME)>
     Xattr Value = <BASENAME>

Non-crypto hash is suitable for this purpose.
Number of Xattrs on a file = Number of Links

Converting GFID to Path
Example GFID: 78e8bce0-a8c9-4e67-9ffb-c4c4c7eff038

1. List all xattrs of GFID file in the brick backend.
2. If Xattr Key starts with “meta”, Split to get parent GFID and collect 
xattr value
3. Convert Parent GFID to path using recursive readlink till path.
4. Join Converted parent dir path and xattr value(basename)

RENAME: Remove old xattr(PGFID1, BN1), Add new xattr(PGFID2, BN2)
UNLINK: If Link count > 1 then Remove xattr(PGFID, BN)

Heal on Lookup
Healing on lookup can be enabled if required, by default we can
disable this option since this may have performance implications
during read.

Enabling the logging
This can be enabled using Volume set option. Option name TBD.

Rebuild Index
Offline activity, crawls the backend filesystem and builds all the 
required xattrs.

Comments and Suggestions Welcome.


On 11/25/2015 10:08 AM, Aravinda wrote:
> regards
> Aravinda
> On 11/24/2015 11:25 PM, Shyam wrote:
>> There seem to be other interested consumers in gluster for the same 
>> information, and I guess we need a god base design to address this on 
>> disk change, so that it can be leveraged in the various use cases 
>> appropriately.
>> Request a few folks to list out how they would use this feature and 
>> also what performance characteristics they expect around the same.
>> - gluster find class of utilties
>> - change log processors
>> - swift on file
>> - inotify support on gluster
>> - Others?
> Debugging utilities for users/admins(Show path for GFIDs displayed in 
> log files)
> Retrigger Sync in Geo-replication(Geo-rep reports failed GFIDs in 
> logs, we can retrigger sync if path is known instead of GFID)
>> [3] is an attempt in XFS to do the same, possibly there is a more 
>> later thread around the same that discusses later approaches.
>> [4] slide 13 onwards talks about how cephfs does this. (see cephfs 
>> inode backtraces)
>> Aravinda, could you put up a design for the same, and how and where 
>> this is information is added etc. Would help review it from other 
>> xlators perspective (like existing DHT).
>> Shyam
>> [3] http://oss.sgi.com/archives/xfs/2014-01/msg00224.html
>> [4] 
>> http://events.linuxfoundation.org/sites/events/files/slides/CephFS-Vault.pdf
>> On 10/27/2015 10:02 AM, Shyam wrote:
>>> Aravinda, List,
>>> The topic is interesting and also relevant in the case of DHT2 where we
>>> lose the hierarchy on a single brick (unlike the older DHT) and so some
>>> of the thoughts here are along the same lines as what we are debating
>>> w.r.t DHT2 as well.
>>> Here is another option that extends the current thought, that I would
>>> like to put forward, that is pretty much inspired from the Linux kernel
>>> NFS implementation (based on my current understanding of the same) 
>>> [1] [2].
>>> If gluster server/brick processes handed out handles, (which are
>>> currently just GFID (or inode #) of the file), that encode pGFID/GFID,
>>> then on any handle based operation, we get the pGFID/GFID for the 
>>> object
>>> being operated on. This solves the first part of the problem where we
>>> are encoding the pGFID in the xattr, and here we not only do that but
>>> further hand out the handle with that relationship.
>>> It also helps when an object is renamed and we still allow the older
>>> handle to be used for operations. Not a bad thing in some cases, and
>>> possibly not the best thing to do in some other cases (say access).
>>> To further this knowledge back to a name, what you propose can be 
>>> stored
>>> on the object itself. Thus giving us a short dentry tree creation
>>> ability of pGFID->name(GFID).
>>> This of course changes the gluster RPC wire protocol, as we need to
>>> encode/send pGFID as well in some cases (or could be done adding 
>>> this to
>>> the xdata payload.
>>> Shyam
>>> [1] http://nfs.sourceforge.net/#faq_c7
>>> [2] https://www.kernel.org/doc/Documentation/filesystems/nfs/Exporting
>>> On 10/27/2015 03:07 AM, Aravinda wrote:
>>>> Hi,
>>>> We have a volume option called "build-pgfid:on" to enable recording
>>>> parent gfid in file xattr. This simplifies the GFID to Path 
>>>> conversion.
>>>> Is it possible to save base name also in xattr along with PGFID? It
>>>> helps in converting GFID to Path easily without doing crawl.
>>>> Example structure,
>>>> dir1 (3c789e71-24b0-4723-92a2-7eb3c14b4114)
>>>>      - f1 (0aa94a0a-62aa-4afc-9d59-eb68ad39f78c)
>>>>      - f2 (f1e7ad00-6500-4284-b21c-d02766ecc336)
>>>> dir2 (6c3bf2ea-9b52-4bda-a1db-01f3ed5e3fed)
>>>>      - h1 (0aa94a0a-62aa-4afc-9d59-eb68ad39f78c)
>>>> Where file f1 and h1 are hardlinks. Note the same GFID.
>>>> Backend,
>>>> .glusterfs
>>>>       - 3c/78/3c789e71-24b0-4723-92a2-7eb3c14b4114
>>>>       - 0a/a9/0aa94a0a-62aa-4afc-9d59-eb68ad39f78c
>>>>       - f1/e7/f1e7ad00-6500-4284-b21c-d02766ecc336
>>>>       - 6c/3b/6c3bf2ea-9b52-4bda-a1db-01f3ed5e3fed
>>>> Since f1 and h1 are hardlinks accross directories, file xattr will 
>>>> have
>>>> two parent GFIDs. Xattr dump will be,
>>>> trusted.pgfid.3c789e71-24b0-4723-92a2-7eb3c14b4114=1
>>>> trusted.pgfid.6c3bf2ea-9b52-4bda-a1db-01f3ed5e3fed=1
>>>> Number shows number of hardlinks per parent GFID.
>>>> If we know GFID of a file, to get path,
>>>> 1. Identify which brick has that file using pathinfo xattr.
>>>> 2. Get all parent GFIDs(using listxattr on backend gfid path
>>>> .glusterfs/0a/a9/0aa94a0a-62aa-4afc-9d59-eb68ad39f78c)
>>>> 3. Crawl those directories to find files with same inode as
>>>> .glusterfs/0a/a9/0aa94a0a-62aa-4afc-9d59-eb68ad39f78c
>>>> Updating PGFID to be done when,
>>>> 1. CREATE/MKNOD - Add xattr
>>>> 2. RENAME - If moved to different directory, Update PGFID
>>>> 3. UNLINK - If number of links is more than 1. Reduce number of link,
>>>> Remove respective parent PGFID
>>>> 4. LINK - Add PGFID if link to different directory, Increment count
>>>> Advantageous:
>>>> 1. Crawling is limited to a few directories instead of full file 
>>>> system
>>>> crawl.
>>>> 2. Break early during crawl when search reaches the hardlinks 
>>>> number as
>>>> of Xattr value.
>>>> Disadvantageous:
>>>> 1. Crawling is expensive if a directory has lot of files.
>>>> 3. This method of conversion will not work if file is deleted.
>>>> We can improve performance of GFID to Path conversion if we record
>>>> Basename also in file xattr.
>>>> trusted.pgfid.3c789e71-24b0-4723-92a2-7eb3c14b4114=f1
>>>> trusted.pgfid.6c3bf2ea-9b52-4bda-a1db-01f3ed5e3fed=h1
>>>> Note: Multiple base names delimited by zerobyte.
>>>> What additional overhead compare to storing only PGFID,
>>>> 1. Space
>>>> 2. Number of xattrs will grow as number of hardlinks
>>>> 3. Max size issue for xattr value?
>>>> 4. Even when renamed within the same directory.
>>>> 5. Updating value of xattr involves parsing in case of multiple
>>>> hardlinks.
>>>> Are there any performance issues except during initial 
>>>> indexing.(Assume
>>>> pgfid and basenames are populated by a separate script)
>>>> Comments and Suggestions Welcome.
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