On 12/19/2016 07:57 AM, Aravinda wrote:
regards
Aravinda
On 12/16/2016 05:47 PM, Xavier Hernandez wrote:
On 12/16/2016 08:31 AM, Aravinda wrote:
Proposal to add one more byte to GFID to store "Type" information.
Extra byte will represent type(directory: 00, file: 01, Symlink: 02
etc)
For example, if a directory GFID is f4f18c02-0360-4cdc-8c00-0164e49a7afd
then, GFID2 will be 00f4f18c02-0360-4cdc-8c00-0164e49a7afd.
Changes to Backend store
------------------------
Existing: .glusterfs/gfid[0:2]/gfid/[2:4]/gfid
Proposed: .glusterfs/gfid2[0:2]/gfid2[2:4]/gfid2[4:6]/gfid2
Advantages:
-----------
- Automatic grouping in .glusterfs directory based on file Type.
- Easy identification of Type by looking at GFID in logs/status output
etc.
- Crawling(Quota/AFR): List of directories can be easily fetched by
crawling `.glusterfs/gfid2[0:2]/` directory. This enables easy
parallel Crawling.
- Quota - Marker: Marker transator can mark xtime of current file and
parent directory. No need to update xtime xattr of all directories
till root.
- Geo-replication: - Crawl can be multithreaded during initial sync.
With marker changes above it will be more effective in crawling.
Please add if any more advantageous.
Disadvantageous:
----------------
Functionality is not changed with the above change except the length
of the ID. I can't think of any disadvantages except the code changes
to accommodate this change. Let me know if I missed anything here.
One disadvantage is that 17 bytes is a very ugly number for
structures. Compilers will add paddings that will make any structure
containing a GFID noticeable bigger. This will also cause troubles on
all binary formats where a GFID is used, making them incompatible. One
clear case of this is the XDR encoding of the gluster protocol.
Currently a GFID is defined this way in many places:
opaque gfid[16]
This seems to make it quite complex to allow a mix of gluster versions
in the same cluster (for example in a middle of an upgrade).
What about this alternative approach:
Based on the RFC4122 [1] that describes the format of an UUID, we can
define a new structure for new GFID's using the same length.
Currently all GFID's are generated using the "random" method. This
means that all GFID have this structure:
xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx
Where N can be 8, 9, A or B, and M is 4.
There are some special GFID's that have a M=0 and N=0, for example the
root GFID.
What I propose is to use a new variant of GFID, for example E or F
(officially marked as reserved for future definition) or even 0 to 7.
We could use M as an internal version for the GFID structure (defined
by ourselves when needed). Then we could use the first 4 or 8 bits of
each GFID as you propose, without needing to extend current GFID
length nor risking to collide with existing GFID's.
If we are concerned about the collision probability (quite small but
still bigger than the current version) because we loose some random
bits, we could use N = 0..7 and leave M random. This way we get 5 more
random bits, from which we could use 4 to represent the inode type.
I think this way everything will work smoothly with older versions
with minimal effort.
What do you think ?
That is really nice suggestion.
To get the crawling advantageous as mentioned above, we need to make
backend store as .glusterfs/N/gfid[0:2]/gfid[2:4]/gfid
That's one possibility. Since N will be 4 bits at most, it won't collide
with currently existing subdirectories that represent 8 bits. Or we
could use M. It all depends on the exact interpretation we give to each
field.
One suggestion I would make is to define it in a way that we use the
minimal amount of bits to represent what we need now but leave space for
future extensions. For example creating a "reserved" value for the field.
Proposal:
Use N = 00xx for special GFID's, like NULL GFID, or the ones currently
used in some places. All these will also have M = 0. All other values of
M will be reserved for future extensions.
Also reserve all other values of N (01xx) for future extensions.
This gives a lot of space to represent many things in the future if
necessary, while keeping current usage compatible with it.
For this particular case we could use N = 0000 and define M as (this is
a mapping of the posix S_IFxxx values):
M = 0000 Current special GFID's
M = 0001 Fifo (S_IFIFO)
M = 0010 Character Device (S_IFCHR)
M = 0100 Directory (S_IFDIR)
M = 0110 Block Device (S_IFBLK)
M = 1000 Regular File (S_IFREG)
M = 1010 Symbolic Link (S_IFLNK)
M = 1100 Socket (S_IFSOCK)
M = xx11 \
M = x1x1 | Reserved for future extensions
M = 1xx1 |
M = 111x /
If we use our own mapping instead of using the same values than IF_Sxxx
macros, we can get a more compact representation if needed.
In this case the directory structure could be
.glusterfs/M/gfid[0:2]/gfid[2:4]/gfid. And use M = 0 to put all current
existing gfid's, or we could leave existing gfid's in their current
location.
Or we could even have .glusterfs/NM/gfid[0:2]/gfid[2:4]/gfid. This would
probably be compatible even with future extensions.
Xavi
Xavi
[1] https://www.ietf.org/rfc/rfc4122.txt
Changes:
---------
- Code changes to accommodate 17 bytes GFID instead of 16 bytes(Read
and Write)
- Migration Tool to upgrade GFIDs in Volume/Cluster
Let me know your thoughts.
_______________________________________________
Gluster-devel mailing list
Gluster-devel@xxxxxxxxxxx
http://www.gluster.org/mailman/listinfo/gluster-devel
