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

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Date: Wed, 6 Mar 2024 09:41:16 +0000
From: John Garry <john.g.garry@...cle.com>
To: Dave Chinner <david@...morbit.com>
Cc: djwong@...nel.org, hch@....de, viro@...iv.linux.org.uk, brauner@...nel.org,
        jack@...e.cz, chandan.babu@...cle.com, axboe@...nel.dk,
        martin.petersen@...cle.com, linux-kernel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        tytso@....edu, jbongio@...gle.com, ojaswin@...ux.ibm.com,
        ritesh.list@...il.com, linux-block@...r.kernel.org
Subject: Re: [PATCH v2 04/14] fs: xfs: Make file data allocations observe the
 'forcealign' flag


>> 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