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

To: Brian Foster <bfoster@xxxxxxxxxx>
Subject: Re: xfsprogs/mkfs: what's the relationship between max trans res and min log size
From: Wang Sheng-Hui <shhuiw@xxxxxxxxxxx>
Date: Tue, 19 May 2015 07:28:05 +0800
Cc: xfs <xfs@xxxxxxxxxxx>, Dave Chinner <dchinner@xxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20150518133439.GA43596@xxxxxxxxxxxxxxx>
References: <555987A2.3050604@xxxxxxxxxxx> <20150518133439.GA43596@xxxxxxxxxxxxxxx>
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On 2015å05æ18æ 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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> 
> 


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