Re: [PATCH v2 04/14] fs: xfs: Make file data allocations observe the 'forcealign' flag

John Garry
2024-03-06T09:41:40Z

>> Please note in case missed, I am mandating extsize hint for forcealign needs
>> to be a power-of-2. It just makes life easier for all the sub-extent
>> zero'ing later on.
>
> That's fine - that will need to be documented in the xfsctl man
> page...
>
>> Also we need to enforce that the AG count to be compliant with the extsize
> ^^^^^ size?

Yes

>
>> hint for forcealign; but since the extsize hint for forcealign needs to be
>> compliant with stripe unit, above, and stripe unit would be compliant wth AG
>> count (right?), then this would be a given.
>
> We already align AG size to stripe unit when a stripe unit is set,
> and ensure that we don't place all the AG headers on the same stripe
> unit.
>
> However, if there is no stripe unit we don't align the AG to
> anything.

> So, yes, AG sizing by mkfs will need to ensure that all
> AGs are correctly aligned to the underlying storage (integer
> multiple of the max atomic write size, right?)...

right, this is really important

>
>>> More below....
>>>
>>>> + } else {
>>>> + args->alignment = 1;
>>>> + }
>>>
>>> Just initialise the allocation args structure with a value of 1 like
>>> we already do?
>>
>> It was being done in this way to have just a single place where the value is
>> initialised. It can easily be kept as is.
>
> I'd prefer it as is, because then the value is always initialised
> correctly and we only override in the special cases....

ok

>>
>> are you saying that low-space allocator can set args->alignment = 1 to be
>> explicit?
>
> Yes.

ok

Thanks,
John