Re: [PATCH 0/7] fio: atomic write support

[John Garry <john.g.garry@oracle.com>]

On 10/09/2024 02:01, Jens Axboe wrote:

+ Dave, who might be interested in this

>> I was preparing a v2 the same as this, but do you think that I should
>> just add a new verify option for atomic writes to ignore the header?
>> Or is just saying "you have selected atomic=1, so I'll just ignore the
>> header for you" ok?
> I'm still not following why atomic writes make this any different. They
> should not. If you have multiple writers writing the same blocks, yeah
> you will get verification errors. This is true for any kind of write, be
> it buffered, dio, or atomic. So unless I'm missing something here, I'd
> just drop that patch.

Some background is that main selling point of atomic writes is that we 
guarantee writes to storage will not be torn for a power failure or 
kernel crash.

Another aspect of atomic writes is that they handle racing writes and 
reads, such that a read racing with a write will see all the data from 
the write or none. Well, SCSI and NVMe guarantee this if using 
RWF_ATOMIC, but it is not formally stated as a feature of RWF_ATOMIC.

It can be argued that having racing reads and writes is an application 
bug. Furthermore, as I understand, even if posix guarantees that regular 
writes are "atomic", it is not the case generally.

So one part of the relevance of atomic writes to fio verify is that we 
can verify that atomic writes "safely" handle racing read and writes. 
For this, the CRC checks would be successful if we have many jobs; 
however header sequence numbers are not. Hence patch 4/7.

I had also been using the verify feature to test atomic writes for power 
failures. In this case, I run a single verify job with --rw=write, power 
fail, and use verify in read mode to prove no invalid data in the file, 
like:

fio --filename=mnt/file --direct=1 --rw=read --bs=8k --iodepth=100 --na
me=iops --numjobs=1 --loops=1 --verify=crc64 --ioengine=libaio
--verify_fatal=1 --group_reporting  --exitall_on_error

This power fail test is what I am mostly interested in.

So my point is that the patch to ignore invalid headers could be 
dropped, but let me know your thoughts.

Thanks,
John