Re: [PATCH] ubifs: Introduce a mount option of force_atime.

[Richard Weinberger <richard@nod.at>]

Am 08.06.2015 um 11:11 schrieb Dongsheng Yang:
> On 06/08/2015 04:44 PM, Richard Weinberger wrote:
>> Am 08.06.2015 um 10:27 schrieb Dongsheng Yang:
>>> Currently, ubifs does not support access time anyway. I understand
>>> that there is a overhead to update inode in each access from user.
>>>
>>> But for the following two reasons, I think we can make it optional
>>> to user.
>>>
>>> (1). More and more flash storage in server are trying to use ubifs,
>>> it is not only for a device such as mobile phone any more, we want
>>> to use it in more and more generic way. Then we need to compete
>>> with some other main filesystems. From this point, access time is
>>> necessary to us, at least as a choice to user currently.
>>
>> Do you have a reference? I know that modern servers use a lot of SSDs
>> which use internally NAND (mostly MLC and TLC).
>> But which systems use RAW NAND where they would care about the atime?
> 
> Hi Richard,
> 
> Thanx for your quick response here.
> 
> http://www.slideshare.net/FujitsuTS/bos-c113a-data-will-change-business-but-will-it-really-change-ict
> I am not sure is that url available to you. But that's what my team is
> focus on. It's about a server-using NAND device.

So, you want to use UBI/UBIFS on NAND attached via PCEe?
Is this SLC NAND? (UBI and UBIFS was designed with SLC in mind).
MLC and TLC are a major challenge for UBI/UBIFS.

>>
>>> (2). The default mount option about atime is relatime currently,
>>> it's much relaxy compared with strictatime. Then we don't update
>>> the inode in any accessing. So the overhead is not too much.
>>> It's really acceptable.
>>
>> Did you consider ext4's lazytime? I can think of something like that
>> for UBIFS too.
> 
> Yes, lazytime is much better in our usecase, from what I know,
> they are trying to implement a lazytime in vfs.
> 
> But what I am doing here is just making the atime possible to user. It
> means the force_atime is not in the same level with relatime,
> strictatime and lazytime. force_atime here is just making our ubifs
> supporting access time in any mode as you chose. If you want to use
> relatime or strictatime, even or lazytime in future, for ubifs, you
> have to enable force_atime at first. otherwise we does not support access atime anyway.

Let's name is "enable_atime" instead of "force_atime".
The question is, how much will regular "atime" and "relatime" hurt the NAND.
Do you have numbers?

Thanks,
//richard