[Lustre-devel] How store HSM metadata in MDT ?

Rick Matthews Richard.Matthews at Sun.COM
Tue Jul 8 05:06:09 PDT 2008

Peter and Lee,
  Lee, you are correct when pointing out the versioning of a file with a
backup copy is a backup style function. One desirable to users of backup ans
some HSM products, but still primarily driven by the "coincidence" that
older copies remain, and reference to them may be desired. (In most 
these references are used to either "restore this copy to that directory" or
"restore this directory tree to its prior state".) So, while primarily a 
function, one that if an HSM is the basis for backup copies may be important
in the future.

  HSM as the basis of backup copies is a desirable trait IMHO. The HSM 
is already
retaining an instance of the file, one which could easily be captured as 
a "backup"
copy. That said, HSM and snapshot seems to bring a better mix, 
particularly to the
user. A snapshot of the file system presents a consistent view, and the 
backup would
only need to include data (metadata) from files previously resident in 
the HSM.

  As for HSM and deduplication, I see the deduplication being an 
optimization tradeoff
with consumed space. In a relatively expensive random access media (like 
disk), deduplication
provides a reduced total data footprint while not affecting the 
retrieval rate significantly.
When the media is sequentially oriented and relatively less expensive to 
have (like tape),
deduplication seems to not make as much sense. So, I see deduplication 
as important on disk
based archive copies, and not all that useful in tape archiving. Of 
course, tape striping
is important, but is still a sequential store/retrieve. Also, if it is 
convenient to deduplicate
full sequential images of a file (while not violating a numbers of 
copies policy), that
should be done on the sequentially oriented media. There may also be 
some policy (sequential
affinity) reasons where even the full image deduplication is not desirable.

  Thank you for letting me participate in this discussion.

Peter Braam wrote:
> Lee - Thank you for this clear explanation.
> If solely the HSM can store multiple versions, we have already some
> difficulties.  One might imagine setting a particular version in the HSM as
> the primary one, meaning that this primary one will be transparently
> restored or that a pre-staging utility will select this by default.
> If the file is fully absent in the file system staging or restoring it will
> work correctly.  However, if a part of the file remains in the file system,
> this HSM versioning becomes complicated because the file will again have to
> remember what HSM versions the fragments belong to, and we are almost back
> where we were.
> I think the emails so far make it clear that we don't want to have one
> Lustre inode be associated with multiple objects in the HSM.
> If the HSM system is used as a backup then the restore operations will have
> user or operator involvement and this objection to storing multiple versions
> in the HSM does not apply. However, the we still don't' want to store a
> pointer to each version in the file system, that belongs in the HSM/backup
> metadata store.
> However, I don't want to end the discussion right here.
> With DMU (or otherwise) we will get file systems where snapshots become
> possible and common, and these snapshots will contain different versions of
> the same file.  The way the namespace distinguishes these is that in the
> pair (fsid, fid) the fsid is different for each snapshot.   So probably the
> id in the HSM should allow for an fsid component.
> Now DMU snapshot versions of one inode share blocks, and this leads to the
> question if/how we can efficiently share blocks in the HSM also.  This
> discussion would probably equally apply to upcoming "dedup" efforts for the
> DMU, which the virtualization and "email attachment" community think are
> very important.
> Rick, Jeff  - how will we handle this?
> Peter
> On 7/6/08 1:24 PM, "Lee Ward" <lee at sandia.gov> wrote:
>> Are you all talking about HSM, really, or simply backup?
>> If backup, read no further.
>> If HSM, then, do you intend that the user be allowed to specify *which*
>> version of the file content is desired?
>> If yes and you also want the standard API and utilities to function,
>> seamlessly, then the version must be exposed in the name space, no? I.e.
>> For any file named "foo" with 3 versions, for instance, there would be
>> foo;1, foo;2, foo;3, and "foo" which is an alias for "foo;1".
>> If no, then, you'll have to craft a special API that will motivate
>> special tools. However, HPSS already has this API and set of tools so
>> what's the point? Wouldn't it be better to just modify HPSS to
>> understand versions?
>> If HSM, then, do you intend that two users might be allowed to work with
>> two, or more, versions of the file content simultaneously?
>> If yes then same problem as above since those two versions might need to
>> be in the same directory, at the same time, right?
>> No matter what you do, you have problems that can't be resolved when
>> mixing a POSIX name space with file versions, I believe. Since POSIX
>> reserves no characters you can't pick a scheme that includes version
>> information in the name without at least being confusing and the API
>> provides no other way to specify the version, no?
>> My personal choice would be to shy off direct version support by the
>> native file system. It doesn't seem to have a reasonable solution
>> without involving the user somehow to specify names or naming schemes.
>> That kind of involvement just begs for a special utility and, once
>> there, relieves the file system of the need to support any but the most
>> recent version itself, anyway.
>> --Lee
>> On Sat, 2008-07-05 at 21:24 -0600, Peter Braam wrote:
>>> On 7/4/08 8:37 AM, "Aurelien Degremont" <aurelien.degremont at cea.fr> wrote:
>>>> Peter Braam a écrit :
>>>>> If there is more than one copy in the archive, it would be preferable if
>>>>> the
>>>>> archive could maintain a mapping from the Lustre fid of the file to the
>>>>> archived copies.  Associated with the FID of the data would then be a list
>>>>> of archived copies, timestamps etc.
>>>> Do you mean that the HSM will be aware of various versions of one same
>>>> file, identified in Lustre by a FID ?
>>>> Or this will be masked by the archiving tool , doing some tricks to
>>>> simulate it ?
>>>>> Can that be done in HPSS?
>>>> HPSS alone cannot do versioning on its files presently.
>>> But your archiving utility that copies from Lustre to HPSS can maintain
>>> database of these objects - no need to store anything in Lustre.
>>>>> If not, policy related operations like purging older files etc will become
>>>>> very complex and not scalable.  For example, a search to find older files
>>>>> in
>>>>> the archive would require an e2scan operation to find the inodes and then
>>>>> the objects in the archive.  If the file system was not available anymore
>>>>> (for whatever reason), it is not even clear that such a purge could still
>>>>> happen.
>>>>> With an archive based database this can be an indexed search in the
>>>>> archive,
>>>>> which is faster and more appropriate.
>>>> By purgin do mean purging in Lustre or in the HSM?
>>> The HSM.
>>>> There's no issue with purging in Lustre because this do not imply the HSM.
>>>> And removal of oldest copies in the HSM could be done asynchronously,
>>>> slowly.
>>> There is a rule in Lustre - no scanning, ever.  This rule will not be broken
>>> by HSM.
>>> So, you have to move your management of ID's of the archvied copies outside
>>> of Lustre, in some database.  This will actually save you time - doing this
>>> in the MDS will be no fun.
>>> The MDS should only get attributes to indicate if and what version of a file
>>> is in the archive and a cursor (maybe other information) in relation with
>>> ongoing restores.
>>> Peter
>>>> I'm not sure I see what you mean here
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>>> Lustre-devel at lists.lustre.org
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Rick Matthews                           email: Rick.Matthews at sun.com
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