Re: [RFC] [PATCHv3 7/9] reiser4: batch discard support: actually implement the FITRIM ioctl handler.

[Edward Shishkin <edward.shishkin@gmail.com>]

On 10/21/2014 08:11 PM, Ivan Shapovalov wrote:
> On Tuesday 21 October 2014 at 20:01:55, Edward Shishkin wrote:	
>> On 10/21/2014 06:42 PM, Ivan Shapovalov wrote:
>>> On Tuesday 21 October 2014 at 18:33:11, Edward Shishkin wrote:	
>>>> On 10/21/2014 06:23 PM, Ivan Shapovalov wrote:
>>>>> On Tuesday 21 October 2014 at 18:21:56, Edward Shishkin wrote:	
>>>>>> [...]
>>>>>>
>>>>>> Sorry, but I don't see any explanation.
>>>>> "given a filesystem with (5% + eps) free space left, not using the reserved
>>>>> space will result in trimming of (eps) blocks at a time."
>>>>>
>>>> This is something that I am not able to parse :)
>>>>
>>>> Ok, it is clear, why we can not fail with -ENOSPC when trying to delete
>>>> a file from a full partition, yes?
>>> Sure. Please note that making reiser4_trim_fs's allocations BA_RESERVED will
>>> not hinder this: delete_mutex is there for a reason.
>>
>> Sure, and this is the reason, because when I hear "let's take a mutex",
>> I start to feel bad :)
>>
>>
>>>> I want to see explanation, why FITRIM ioclt can not finish the work when
>>>> there is no free space on disk.
>>> Suppose we do not use reserved space for reiser4_trim_fs's allocations.
>>> Let's analyze those two cases:
>>>
>>> 1. There is <= 5% free space on disk.
>>> Initial grabbing fails, nothing can be trimmed.
>>> This is wrong.
>>>
>>> 2. There is 5% + X (where X is some small number) free space on disk.
>>> We can grab only X blocks at a time, so a total of ((SIZE * 5% / X) + 1)
>>> transactions will be created. BTW, if X < erase unit, nothing can be trimmed.
>>> This is ineffective.
>>>
>>> Hope this makes sense.
>>
>> This is already much better, thanks!
>> Then I should say that the whole idea to reserve % of free space is
>> incorrect.
>> Try to reserve 1% of total partition size with BA_CAN_COMMIT for an
>> iteration.
>> If it fails, then try to reserve 1 block with BA_CAN_COMMIT.
> Hm? Both described cases are still broken.
>
> In #1 (free space <= 5%) nothing changes; reiser4_trim_fs will never be able
> to do its task without using reserved space.
>
> In updated #2 (5% < free space < 6%), a transaction per every block (!)
> will be created, and none of them will trim anything (1 block < erase unit).


If (5% < free_space < 5% + erase_unit), then fail with ENOSPC.
Why not? This is not the unlink case...