[Gluster-devel] GFID to Path Conversion
Shyam
srangana at redhat.com
Tue Jan 5 21:19:22 UTC 2016
On 12/09/2015 12:47 AM, Aravinda wrote:
> 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)
(current thought) DHT2 would extend the manner in which name,pGFID is
stored for files, for directories. So reverse path walking would
leverage the same mechanism as explained below.
Of course, as this would involve MDS hopping, the intention would be to
*not* use this in IO critical paths, and rather use this in the tool set
that needs reverse path walks to provide information to admins.
>
> 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>
I would think we should keep the xattr name constant, and specialize the
value, instead of encoding data in the xattr value itself. The issue is
of course multiple xattr name:value pairs where name is constant is not
feasible and needs some thought.
>
> 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
Here is where we get into a bit of a problem, if a file has links. Which
path to follow would be a dilemma. We could return all paths, but tools
like glusterfind or backup related, would prefer a single file. One of
the thoughts is, if we could feed a pGFID:GFID pair as input, this still
does not solve a file having links within the same pGFID.
Anyway, something to note or consider.
>
> 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.
This is the part which should/would change with DHT2 in my opinion. Sort
of repeating step (2) here instead of a readlink.
> 4. Join Converted parent dir path and xattr value(basename)
>
> Recording
> ---------
> MKNOD/CREATE/LINK/SYMLINK: Add new Xattr(PGFID, BN)
Most of these operations as they exist today are not atomic, i.e we
create the file and then add the xattrs and then possibly hardlink the
GFID, so by the time the GFID makes it's presence, the file is all ready
and (maybe) hence consistent.
The other way to look at this is that we get the GFID representation
ready, and then hard link the name into the name tree. Alternately we
could leverage O_TMPFILE to create the file encode all its inode
information and then bring it to life in the namespace. This is
orthogonal to this design, but brings in needs to be consistent on failures.
Either way, if a failure occurs midway, we have no way to recover the
information for the inode and set it right. Thoughts?
> 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.
Frequency of the rebuild? I would assume this would be run when the
option is enabled, and later almost never, unless we want to recover
from some inconsistency in the data (how to detect the same would be an
open question).
Also I think once this option is enabled, we should prevent disabling
the same (or at least till the packages are downgraded), as this would
be a hinge that multiple other features may depend on, and so we
consider this an on-disk change that is made once, and later maintained
for the volume, rather than turn on/off.
Which means the initial index rebuild would be a volume version
conversion from current to this representation and may need aditional
thoughts on how we maintain volume versions.
>
> 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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