[Lustre-devel] rename in Changelog

[Mikhail Pershin <mike.tappro@gmail.com>]

On Wed, 25 Apr 2012 20:36:08 +0400, Nathan Rutman  
<Nathan_Rutman@xyratex.com> wrote:

> it wasn't done that way in the first place because the record size in an  
> llog is fixed,
> so any size increase is multiplied by the number of records, so fewer  
> records
> can be stored.  Splitting the rename into two was the unfortunate  
> casualty of that goal.

This is true if we are using the same record for all operations and Lai  
did so by adding extra field. But I'd note that fixed size is not  
mandatory thing but good to have. Moreover, changelog records are  
naturally not fixed size because contain name which size is not fixed. So  
I suppose rename was done in two parts just as simpler way to go because  
required no additional changes in processing, etc. Also, IIRC, CL_EXT  
appeared even later than CL_RENAME itself.

To avoid space consuming we can just introduce extended  
'changelog_ext_rec' for rename. It is not the problem to create such  
changelogs but requires processing tools to be aware about that, is that a  
big problem or acceptable?

>
> On Apr 25, 2012, at 2:05 AM, Lai Siyao wrote:
>
>> Rename record in Changelog is different from other operations, it's  
>> split into
>> two records: RNMFRM and RNMTO. This makes Changelog analysis hard
>> because these two records may not be consecutive and several renames
>> may occur at the same time.
>>
>> I'm not clear why it's designed to be so, except that RNMTO is needed
>> because for DNE (distributed namespace) the rename target may reside on
>> another MDS, and a separate RNMTO record is needed. But even with this
>> it's fine to store all information in a RENME record, but leave the  
>> information of
>> whether rename removes the last hardlink of the target file (if it  
>> exists) in
>> RNMTO record.
>>
>> I tried to add a field spfid in struct changelog_rec to store source  
>> parent fid,
>> and pack both source (if has) and target names into record. Normally the
>> record size if sizeof(fid) larger than before, and it can be  
>> differentiated
>> according to version. The test result looks good, but I want to know  
>> whether
>> anyone oppose to this? If not, I'll make the change and make it  
>> changelog
>> version 2.
>>
>> Cheers,
>> - Lai
>>
>>
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