[lustre-discuss] [HPDD-discuss] possible to read orphan ost objects on live filesystem?

[Dilger, Andreas]

On 2015/09/01, 7:59 AM, "lustre-discuss on behalf of Chris Hunter"
<lustre-discuss-bounces at lists.lustre.org on behalf of
chris.hunter at yale.edu> wrote:

>Hi Andreas,
>Thanks for your help.
>
>If you have a striped lustre file with "holes" (ie. one chunk is gone
>due hardware failure, etc.) are the remaining file chunks considered
>orphan objects ?
>
>AFAIK the online lfsck tool will scrub orphan objects. When mounting a
>OST on our oss server, I see syslog messages such as:
>> Aug 31 23:20:45 oss1 kernel: Lustre: test-OST0002: deleting orphan
>>objects from 0x0:228989008 to 0x0:228989127
>
>Which leads me to believe these OST objects are subject to removal.
>However I don't know what exactly are orphan objects.

These "orphan objects" are just precreated OST objects that were never
allocated to MDS files before the MDS or OSS crashed (or were allocated
before the MDS crashed but the client didn't complete recovery).  They are
unrelated to the problem you describe.

Cheers, Andreas

>On 09/01/2015 12:58 AM, Dilger, Andreas wrote:
>> On 2015/08/31, 3:46 PM, "HPDD-discuss on behalf of Chris Hunter"
>> <hpdd-discuss-bounces at lists.01.org on behalf of chris.hunter at yale.edu>
>> wrote:
>>
>>> I am recovering from lustre OST failure and subsequent file corruption.
>>> We have striped files each with 1 missing chunk. I would like to dump
>>> the remaining file chunks from the OST. We have some tools (eg.
>>>debugfs)
>>> to grab the good chunks.
>>>
>>> My question, if we put the filesystem into production (ie. users start
>>> writing new files). What will happen to these good chunks ?
>>>
>>> Does lustre consider these "orphan" inodes (and lfsck deletes them) ?
>>
>> Since it was the OST that failed and not the MDT, then the remaining OST
>> objects would not be removed.
>>
>> You can read the good chunks of such a file using:
>>
>>    dd if=<file> of=<file>.new bs=1M conv=sync,noerror count=<size_in_mb>
>>    truncate --size=<size_in_bytes> <file>.new
>>
>> The "conv=sync,noerror" allows reading from the file without failing
>> for the read errors returned from the missing stripe.  However, this
>> also prevents the dd from stopping when it hits the end of file, so
>> the number of chunks to be read needs to be specified.
>>
>> Cheers, Andreas
>>
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Cheers, Andreas
-- 
Andreas Dilger

Lustre Software Architect
Intel High Performance Data Division