Re: Is it possible that certain physical disk doesn't implement flush correctly?

[Andrei Borzenkov <arvidjaar@gmail.com>]

31.03.2019 14:27, Alberto Bursi пишет:
> 
> On 30/03/19 13:31, Qu Wenruo wrote:
>> Hi,
>>
>> I'm wondering if it's possible that certain physical device doesn't
>> handle flush correctly.
>>
>> E.g. some vendor does some complex logical in their hdd controller to
>> skip certain flush request (but not all, obviously) to improve performance?
>>
>> Do anyone see such reports?
>>
>> And if proves to happened before, how do we users detect such problem?
>>
>> Can we just check the flush time against the write before flush call?
>> E.g. write X random blocks into that device, call fsync() on it, check
>> the execution time. Repeat Y times, and compare the avg/std.
>> And change X to 2X/4X/..., repeat above check.
>>
>> Thanks,
>> Qu
>>
>>
> 
> Afaik HDDs and SSDs do lie to fsync()
> 
> unless the write cache is turned off with hdparm,

I know at least one case of SSD that are claimed to flush cache in case
of power loss. I can dig up details if anyone is interested.

> 
> hdparm -W0 /dev/sda
> 
> similarly to RAID controllers.
> 
> see below
> 
> https://brad.livejournal.com/2116715.html
> 
> https://queue.acm.org/detail.cfm?id=2367378
> 
> 
> -
>