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From: Goldwyn Rodrigues <rgoldwyn@suse.de>
To: Matthew Wilcox <willy@infradead.org>
Cc: linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 01/15] btrfs: create a mount option for dax
Date: Wed, 27 Mar 2019 07:26:08 -0500	[thread overview]
Message-ID: <20190327122608.qa3z7h2wgnaiiugt@merlin> (raw)
In-Reply-To: <20190327120031.GR10344@bombadil.infradead.org>

On  5:00 27/03, Matthew Wilcox wrote:
> On Wed, Mar 27, 2019 at 06:00:52AM -0500, Goldwyn Rodrigues wrote:
> > On 12:10 26/03, Matthew Wilcox wrote:
> > > On Tue, Mar 26, 2019 at 02:02:47PM -0500, Goldwyn Rodrigues wrote:
> > > > This sets S_DAX in inode->i_flags, which can be used with
> > > > IS_DAX().
> > > > 
> > > > The dax option is restricted to non multi-device mounts.
> > > > dax interacts with the device directly instead of using bio, so
> > > > all bio-hooks which we use for multi-device cannot be performed
> > > > here. While regular read/writes could be manipulated with
> > > > RAID0/1, mmap() is still an issue.
> > > > 
> > > > Auto-setting free space tree, because dealing with free space
> > > > inode (specifically readpages) is a nightmare.
> > > > Auto-setting nodatasum because we don't get callback for writing
> > > > checksums after mmap()s.
> > > 
> > > Congratulations on getting the bear to dance.  But why?
> > 
> > Why not ? ;)
> 
>  18 files changed, 662 insertions(+), 77 deletions(-)
> 
> I want to know what advantage we're getting for that.

Direct device mmap'd files, faster access, lower pagecache usage...
all the advantages you would get from a dax device.

> 
> > > To me, the point of btrfs is all the cool stuff it does with built-in
> > > checksumming and snapshots and RAID and so on.  DAX doesn't let you do
> > > any of that, so why would somebody want to use btrfs to manage DAX?
> > 
> > There are users who are asking for advantages of dax on btrfs.
> 
> There are also people asking for perpetual motion machines.  Do these
> users understand the tradeoffs?

The only major feature they would not be able to use is
multi-device, which will be relayed at mount time.

> 
> > I have tried to make it work with snapshots in this series.
> > Checksumming should be possible, but would require some more hacks. I am
> > looking into it.
> > multi-device is an issue for mmap() and I don't think we can work around
> > it.
> > 
> > I agree there is a price to pay to use dax, but I am sure the users
> > would know about that.
> 
> I really doubt it, to be honest.

Care to elaborate?

-- 
Goldwyn

  reply	other threads:[~2019-03-27 12:26 UTC|newest]

Thread overview: 48+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20190326190301.32365-1-rgoldwyn@suse.de>
2019-03-26 19:02 ` [PATCH 01/15] btrfs: create a mount option for dax Goldwyn Rodrigues
2019-03-26 19:10   ` Matthew Wilcox
2019-03-27 11:00     ` Goldwyn Rodrigues
2019-03-27 12:00       ` Matthew Wilcox
2019-03-27 12:26         ` Goldwyn Rodrigues [this message]
2019-03-27 23:31         ` Goldwyn Rodrigues
2019-03-27 17:38     ` Adam Borowski
2019-03-28 14:49   ` David Sterba
2019-03-28 17:28   ` David Sterba
2019-03-28 17:57     ` Darrick J. Wong
2019-04-01 20:43     ` Goldwyn Rodrigues
2019-03-26 19:02 ` [PATCH 02/15] btrfs: Carve out btrfs_get_extent_map_write() out of btrfs_get_blocks_write() Goldwyn Rodrigues
2019-03-26 19:02 ` [PATCH 03/15] btrfs: basic dax read Goldwyn Rodrigues
2019-03-26 19:02 ` [PATCH 04/15] dax: Introduce IOMAP_F_COW for copy-on-write Goldwyn Rodrigues
2019-03-27 17:54   ` Darrick J. Wong
2019-03-27 18:58     ` Goldwyn Rodrigues
2019-03-28 14:45       ` Darrick J. Wong
2019-04-01  4:38   ` Dave Chinner
2019-04-01 21:41     ` Goldwyn Rodrigues
2019-04-01 23:06       ` Dave Chinner
2019-04-03  1:56         ` Goldwyn Rodrigues
2019-04-03  3:20           ` Dave Chinner
2019-04-07  7:26     ` Christoph Hellwig
2019-03-26 19:02 ` [PATCH 05/15] btrfs: return whether extent is nocow or not Goldwyn Rodrigues
2019-03-31 18:42   ` Nikolay Borisov
2019-03-26 19:02 ` [PATCH 06/15] btrfs: Rename __endio_write_update_ordered() to btrfs_update_ordered_extent() Goldwyn Rodrigues
2019-03-26 19:02 ` [PATCH 07/15] btrfs: add dax write support Goldwyn Rodrigues
2019-03-28 14:53   ` Darrick J. Wong
2019-04-01 20:39     ` Goldwyn Rodrigues
2019-03-26 19:02 ` [PATCH 08/15] dax: add dax_iomap_cow to copy a mmap page before writing Goldwyn Rodrigues
2019-03-28 15:41   ` Darrick J. Wong
2019-03-26 19:02 ` [PATCH 09/15] btrfs: add dax mmap support Goldwyn Rodrigues
2019-03-28 15:45   ` Darrick J. Wong
2019-03-26 19:02 ` [PATCH 10/15] btrfs: Add dax specific address_space_operations Goldwyn Rodrigues
2019-03-26 19:02 ` [PATCH 11/15] fs: dedup file range to use a compare function Goldwyn Rodrigues
2019-03-28 17:04   ` Darrick J. Wong
2019-04-01 20:36     ` Goldwyn Rodrigues
2019-03-26 19:02 ` [PATCH 12/15] btrfs: trace functions for btrfs_iomap_begin/end Goldwyn Rodrigues
2019-03-26 19:02 ` [PATCH 13/15] btrfs: handle dax page zeroing Goldwyn Rodrigues
2019-03-26 19:03 ` [PATCH 14/15] btrfs: Disable dax-based defrag and send Goldwyn Rodrigues
2019-03-26 19:03 ` [PATCH 15/15] btrfs: Writeprotect mmap pages on snapshot Goldwyn Rodrigues
2019-03-28 15:48   ` Darrick J. Wong
2019-03-26 19:09 ` [PATCH v2 00/15] btrfs dax support Goldwyn Rodrigues
2019-03-27 20:14   ` Adam Borowski
2019-03-27 23:26     ` Goldwyn Rodrigues
2019-03-28 10:24       ` [PATCH] btrfs: allow MAP_SYNC mmap Adam Borowski
2019-03-28 10:42         ` Adam Borowski
2019-04-01 20:08         ` Goldwyn Rodrigues

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