linux-kernel - Re: [PATCH v6 4/7] xfs: Support FS_XFLAG

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Message-ID: <e934a4aa-0ad9-44ad-a233-2090d3f89805@oracle.com>
Date: Thu, 3 Oct 2024 14:19:09 +0100
From: John Garry <john.g.garry@...cle.com>
To: Christoph Hellwig <hch@....de>
Cc: axboe@...nel.dk, brauner@...nel.org, djwong@...nel.org,
        viro@...iv.linux.org.uk, jack@...e.cz, dchinner@...hat.com,
        cem@...nel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, hare@...e.de,
        martin.petersen@...cle.com, catherine.hoang@...cle.com,
        mcgrof@...nel.org, ritesh.list@...il.com, ojaswin@...ux.ibm.com
Subject: Re: [PATCH v6 4/7] xfs: Support FS_XFLAG_ATOMICWRITES

On 03/10/2024 14:02, Christoph Hellwig wrote:
> On Thu, Oct 03, 2024 at 01:48:41PM +0100, John Garry wrote:
>> On 30/09/2024 13:54, John Garry wrote:
>>> @@ -352,11 +352,15 @@ xfs_sb_has_compat_feature(
>>>    #define XFS_SB_FEAT_RO_COMPAT_RMAPBT   (1 << 1)		/* reverse map btree */
>>>    #define XFS_SB_FEAT_RO_COMPAT_REFLINK  (1 << 2)		/* reflinked files */
>>>    #define XFS_SB_FEAT_RO_COMPAT_INOBTCNT (1 << 3)		/* inobt block counts */
>>> +#define XFS_SB_FEAT_RO_COMPAT_ATOMICWRITES (1 << 31)	/* atomicwrites enabled */
>>> +
>> BTW, Darrick, as you questioned previously, this does make xfs/270 fail...
>> until the change to a not use the top bit.
> With the large block size based atomic writes we shoudn't even need
> a feature flag, or am I missing something?

It really does not add much ATM.

It originated back with you mentioned that we need a generic common way 
to enable atomic writes.

And then we had forcealign, which was tightly-coupled to enabling atomic 
writes, e.g. enforce forcealign enabled and power-of-2 extsize.

But, apart from leveraging larger FS block size for atomic writes, I 
still think that we will want forcealign or similar.

I don't have full tests results yet, but we already know that we see 
performance degradation in some scenarios when going to a 4K -> 16K FS 
block size. We're testing MySQL, and Redo Log performance/efficiency 
significantly degrades with 16K FS block size.

Thanks,
John