Hi,
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.
($BRICK_ROOT/.glusterfs/78/e8/78e8bce0-a8c9-4e67-9ffb-c4c4c7eff038)
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)
Recording
---------
MKNOD/CREATE/LINK/SYMLINK: Add new Xattr(PGFID, BN)
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.
regards
Aravinda
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.
2. Updating PGFID when CREATE/MKNOD/RENAME/UNLINK/LINK
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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