From: Kent Overstreet <kent.overstreet@gmail.com>
To: Vojtech Pavlik <vojtech@suse.com>
Cc: James Pharaoh <james@pharaoh.uk>, linux-bcache@vger.kernel.org
Subject: Re: Extra write mode to close RAID5 write hole (kind of)
Date: Fri, 28 Oct 2016 03:52:49 -0800 [thread overview]
Message-ID: <20161028115249.6myzx2ae24n2w4v7@kmo-pixel> (raw)
In-Reply-To: <20161026223158.GA9283@suse.com>
On Thu, Oct 27, 2016 at 12:31:58AM +0200, Vojtech Pavlik wrote:
> In case you're using mdraid for the RAID part on a reasonably recent
> Linux kernel, there is no write hole. Linux mdraid implements barriers
> properly even on RAID5, at the cost of performance - mdraid waits for a
> barrier to complete on all drives before submitting more i/o.
That's not what the raid 5 hole is. The raid 5 hole comes from the fact that
it's not possible to update the p/q blocks atomically with the data blocks, thus
there is a point in time when they are _inconsistent_ with the rest of the
stripe, and if used will lead to reconstructing incorrect data. There's no way
to fix this with just flushes.
next prev parent reply other threads:[~2016-10-28 11:52 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-10-26 15:20 Extra write mode to close RAID5 write hole (kind of) James Pharaoh
2016-10-26 22:31 ` Vojtech Pavlik
2016-10-27 21:46 ` James Pharaoh
2016-10-28 11:52 ` Kent Overstreet [this message]
2016-10-28 13:07 ` Vojtech Pavlik
2016-10-28 13:13 ` Kent Overstreet
2016-10-28 16:55 ` Vojtech Pavlik
2016-10-28 16:58 ` James Pharaoh
2016-10-28 17:07 ` James Pharaoh
2016-10-29 0:58 ` Kent Overstreet
2016-10-29 19:58 ` James Pharaoh
2016-10-28 11:59 ` Kent Overstreet
2016-10-28 17:02 ` James Pharaoh
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