Re: 回复：ubifs:Questions About Garbage Collection

[Richard Weinberger <richard@nod.at>]

Am 23.06.2016 um 19:20 schrieb Michal Suchanek:
> Hello,
> 
> On 19 June 2016 at 19:05, Richard Weinberger <richard@nod.at> wrote:
>> Am 19.06.2016 um 18:14 schrieb 辉少:
>>> Thanks for your kind reply！Do you mean that in synchronous/asynchronous mode GC is also finished in synchronous
>>> /asynchronous way as writting is? A more question is，are all dirty spaces collected in the same time in synchronous mode(at this point the caller will be blocked for a long time) while in asynchronous mode dirty spaces will be collected little by little in asynchronous mode when i do not  notice？Thank you！
>>
>> Hmm, I don't fully understand your first question, can you elaborate?
>> As for dirty space, GC will not collect all dirty space. UBIFS tells GC how much free space an operation
>> will take. UBIFS calls this budget. The GC will try to produce as much free space as needed.
>>
> 
> I guess the issue here is that UBIFS appears to do this
> 
>  1) write files while there is space
>  2) when space is 0 perform a GC run as part of write operation
> 
> when writes are synchronous the GC run is timed as part of the write.

Exactly. :-)

> If GC was asynchronous it would not appear as part of the synchronous
> write operation and would not cause as much write time jitter. When
> mounting a filesystem synchronous having GC synchronous is likely not
> what the user asked for. It's part of the internal fs bookkeeping and
> not part of the operation the user wants to perform synchronously.
> 
> For GC to run asynchronously it would have to start before the
> available space is 0 so writes can still happen.

How would the GC know how much space you need in future?
Sure, we would run the GC every X seconds/minutes/whatever but then the flash will wear out
faster since GC will always run and not only when you really need space.

> Is it possible to set a parameter somewhere that would make UBIFS do this?

Do we really need this? If your application behaves
correctly there is no need to mount the filesystem synchronous.

Thanks,
//richard