Re: chain_fsetxattr extra chunk removal

[Loic Dachary <loic@dachary.org>]

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On 02/11/2013 06:13 AM, Yehuda Sadeh wrote:
> On Thu, Feb 7, 2013 at 12:59 PM, Loic Dachary <loic@dachary.org> wrote:
>> Hi,
>>
>> While writing unit tests for chain_xattr.cc I tried to understand how to create the conditions to trigger this part of the chain_fsetxattr function:
>>
>>   /* if we're exactly at a chunk size, remove the next one (if wasn't removed
>>      before) */
>>   if (ret >= 0 && chunk_size == CHAIN_XATTR_MAX_BLOCK_LEN) {
>>     get_raw_xattr_name(name, i, raw_name, sizeof(raw_name));
>>     int r = sys_fremovexattr(fd, raw_name);
>>     if (r < 0 && r != -ENODATA)
>>       ret = r;
>>   }
>>
>> I suspect this cleans up extra empty attributes created as a side effect of a previous version of the function. Or I just don't understand the case it addresses.
>>
>> I'd very much appreciate a hint :-)
>>
> 
> Well, the code has changed a bit, but originally when a chain was
> overwritten we didn't bother to remove the xattrs tail. When we read
> the chain we stop either when we got a short xattr, or when the next
> xattr in the chain didn't exist.  So when writing an xattr that was
> perfectly aligned with the block len we had to remove the next xattr
> in order make sure that readers will not over-read. I'm not too sure
> whether that still the case, Sam might have a better idea.
> In any case, it might be a good idea to test the case where we have a
> big xattr that spans across multiple blocks (e.g., > 3) and being
> overwritten by a short xattr. Probably also need to test it with
> different combinations of aligned and non-aligned block sizes.

I understand now and I'll modify the pull request https://github.com/ceph/ceph/pull/40 accordingly.

Thanks :-)

> 
> Thanks,
> Yehuda
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-- 
Loïc Dachary, Artisan Logiciel Libre


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