xfsprogs/mkfs: what's the relationship between max trans res and min log size

[Wang Sheng-Hui]

On 2015年05月18日 21:34, Brian Foster wrote:
> On Mon, May 18, 2015 at 02:33:06PM +0800, Wang Sheng-Hui wrote:
>> Hi guys,
>>
>>
>> In maxtrres.c/max_trans_res, it calls xfs_log_calc_minimum_size to compute the
>>               maximum transaction reservation
>>
>> But for xfs_log_calc_minimum_size, it's used to calculate the
>>               minimum valid log size
>>
>>
>> The naming max/minimum seems confusing.
>>
>> Why are they named in such way? What's the relationship between max trans res
>> and minimun log size, (max trans res) <= (minimun log size)?
>>
>>
>> Any explanations are welcomed!
>>
> 
>>From the comment in xfs_log_calc_minimum_size():
> 
>         /*
>          * Two factors should be taken into account for calculating the minimum
>          * log space.
>          * 1) The fundamental limitation is that no single transaction can be
>          *    larger than half size of the log.
>          *
>          *    From mkfs.xfs, this is considered by the XFS_MIN_LOG_FACTOR
>          *    define, which is set to 3. That means we can definitely fit
>          *    maximally sized 2 transactions in the log. We'll use this same
>          *    value here.
> 	 ...
> 	 */
> 
> So the log must be at least the size of two single transactions (it
> looks like the code is conservative and uses a factor of 3). This is a
> limitation on the minimum allowable log size for the fs. Transactions
> are of variable size, so to determine what that means in terms of actual
> size we must calculate the maximum possible (i.e., worst case)
> transaction size for the particular fs geometry and then use that to
> calculate the minimum total log size.

Thanks, Brian.

Regards,
Sheng-Hui
> 
> Brian
> 
>>
>> Thanks,
>> Sheng-Hui
>>
>>
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>