[Gluster-devel] GFID2 - Proposal to add extra byte to existing GFID

Tue Apr 11 09:29:49 UTC 2017

Comments inline.

On Mon, Dec 19, 2016 at 1:47 PM, Xavier Hernandez <xhernandez at datalab.es>
wrote:

> 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-8c00-0164e4
>>>> 9a7afd
>>>> 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.
>>>>
>>>
Above two will be good enough points to bump up the priority for the
feature.

> - Crawling(Quota/AFR): List of directories can be easily fetched by
>>>>   crawling `.glusterfs/gfid2[0:2]/` directory. This enables easy
>>>>   parallel Crawling.
>>>>
>>>
With the current design, we still have to do a distributed readdir() to get
all
the entries in the directory. This layout change, along with proposed
DHT2/EHT/DHT2+ (name for me doesn't matter here) layout, where directory
entries would be created in just one place should enhance the performance
overall.

> - 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).
>>>
>>
Totally agree with Xavier here. Not in support of adding one more byte.

>
>>> 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.
>
>
I would go with only 'M' being considered for current layout and keeping N
for future developments. Even though we are not considering 'N' internally,
we can keep directory name as '00MM' (zero zero M M). so that backend
layout would be compatible to consider N later if required.

One major thing is we need a solid plan for migration from current layout
to newer layout.

Regards,
Amar

> 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.
>>>>
>>>>
>>>
>>
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