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From: Zygo Blaxell <ce3g8jdj@umail.furryterror.org>
To: "Holger Hoffstätte" <holger@applied-asynchrony.com>
Cc: Hans van Kranenburg <hans@knorrie.org>,
	linux-btrfs <linux-btrfs@vger.kernel.org>
Subject: Re: Q: what exactly does SSD mode still do?
Date: Sat, 28 Mar 2020 15:35:20 -0400
Message-ID: <20200328193520.GZ13306@hungrycats.org> (raw)
In-Reply-To: <116cfdc1-410a-5e09-2fb2-5da2c0fa428a@applied-asynchrony.com>

On Fri, Mar 27, 2020 at 11:29:52AM +0100, Holger Hoffstätte wrote:
> On 3/26/20 11:21 PM, Hans van Kranenburg wrote:
> > 2) Metadata "cluster allocator" write behavior:
> > 
> > *empty_cluster = SZ_64K  # nossd
> > *empty_cluster = SZ_2M  # ssd
> > 
> > This happens in extent-tree.c.
> 
> 2M used to be a common erase block size on SSDs. Or maybe it's just
> a nice round number..  ¯\(ツ)/¯

As a side-effect, 2M write clusters close the write hole on raid5/6 if you
have an array that is a power of 2 data disks wide.  This capability is
wasted when it's only available through the 'ssd' mount option.

The behavior could be quite useful if it was properly integrated with
the raid5/6 stuff:  set *empty_cluster = block group data width, make
sure it's aligned to raid5/6 stripe boundaries, and use it for both data
and metadata.

It works by effectively making partially-filled clusters read-only.
If we can guarantee that clusters are aligned to raid5/6 data/parity block
boundaries, then btrfs can't allocate new data in partially filled raid5/6
stripes, so it won't break the parity relation and won't have write hole.

> cheers,
> Holger
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=08635bae0b4ceb08fe4c156a11c83baec397d36d
> 
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ba8a9d07954397f0645cf62bcc1ef536e8e7ba24
> 

  reply index

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-26 18:16 Holger Hoffstätte
2020-03-26 22:21 ` Hans van Kranenburg
2020-03-27 10:29   ` Holger Hoffstätte
2020-03-28 19:35     ` Zygo Blaxell [this message]
2020-03-28 21:31       ` Hans van Kranenburg

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