Re: [PATCH v6 00/10] block atomic writes

John Garry
2024-04-10T08:35:25Z

On 08/04/2024 18:50, Luis Chamberlain wrote:
> I agree that when you don't set the sector size to 16k you are not forcing the
> filesystem to use 16k IOs, the metadata can still be 4k. But when you
> use a 16k sector size, the 16k IOs should be respected by the
> filesystem.
>
> Do we break BIOs to below a min order if the sector size is also set to
> 16k? I haven't seen that and its unclear when or how that could happen.

AFAICS, the only guarantee is to not split below LBS.

>
> At least for NVMe we don't need to yell to a device to inform it we want
> a 16k IO issued to it to be atomic, if we read that it has the
> capability for it, it just does it. The IO verificaiton can be done with
> blkalgn [0].
>
> Does SCSI*require* an 16k atomic prep work, or can it be done implicitly?
> Does it need WRITE_ATOMIC_16?

physical block size is what we can implicitly write atomically. So if
you have a 4K PBS and 512B LBS, then WRITE_ATOMIC_16 would be required
to write 16KB atomically.

>
> [0]https://urldefense.com/v3/__https://github.com/dagmcr/bcc/tree/blkalgn__;!!ACWV5N9M2RV99hQ!I0tfdPsEq9vdHMSC7JVmVDHCb5w6invjudW7pZW50v3mZ7dWMMf0cBtY_BQlZZmYSjLzPQDZoLO7-K6MQQ$
>
>> So just increasing the inode block size / FS block size does not
>> really change anything, in itself.
> If we're breaking up IOs when a min order is set for an inode, that
> would need to be looked into, but we're not seeing that.

In practice you won't see it, but I am talking about guarantees not to
see it.

>
>>> Do untorn writes actually exist in SCSI? I was under the impression
>>> nobody had actually implemented them in SCSI hardware.
>> I know that some SCSI targets actually atomically write data in chunks >
>> LBS. Obviously atomic vs non-atomic performance is a moot point there, as
>> data is implicitly always atomically written.
>>
>> We actually have an mysql/innodb port of this API working on such a SCSI
>> target.
> I suspect IO verification with the above tool should prove to show the
> same if you use a filesystem with a larger sector size set too, and you
> just would not have to do any changes to userspace other than the
> filesystem creation with say mkfs.xfs params of -b size=16k -s size=16k

Ok, I see

>
>> However I am not sure about atomic write support for other SCSI targets.
> Good to know!
>
>>>> We saw untorn writes as not being a property of the file or even the inode
>>>> itself, but rather an attribute of the specific IO being issued from the
>>>> userspace application.
>>> The problem is that keeping track of that is expensive for buffered
>>> writes. It's a model that only works for direct IO. Arguably we
>>> could make it work for O_SYNC buffered IO, but that'll require some
>>> surgery.
>> To me, O_ATOMIC would be required for buffered atomic writes IO, as we want
>> a fixed-sized IO, so that would mean no mixing of atomic and non-atomic IO.
> Would using the same min and max order for the inode work instead?

Maybe, I would need to check further.

Thanks,
John