From: Duncan <1i5t5.duncan@cox.net>
To: linux-btrfs@vger.kernel.org
Subject: Re: Recommended why to use btrfs for production?
Date: Thu, 9 Jun 2016 06:16:01 +0000 (UTC) [thread overview]
Message-ID: <pan$f0f75$1ba12fd1$de89e173$5dda67a@cox.net> (raw)
In-Reply-To: dd952b29-1bea-59e1-1857-418b86951b47@gmail.com
Austin S. Hemmelgarn posted on Fri, 03 Jun 2016 10:21:12 -0400 as
excerpted:
> As far as BTRFS raid10 mode in general, there are a few things that are
> important to remember about it:
> 1. It stores exactly two copies of everything, any extra disks just add
> to the stripe length on each copy.
I'll add one more, potentially very important, related to this one:
Btrfs raid mode (any of them) works in relation to individual chunks,
*NOT* individual devices.
What that means for btrfs raid10 in combination with the above exactly
two copies rule, is that it works rather differently than a standard
raid10, which can tolerate loss of two devices as long as they're from
the same mirror set, as the other mirror set will then still be whole.
Because with btrfs raid10 the mirror sets are dynamic per-chunk, loss of
a second device close to assures loss of data, because the very likely
true assumption is that both mirror sets will be affected for some
chunks, but not others.
By using a layered approach, btrfs raid1 on top (for its error correction
from the other copy feature) of a pair of mdraid0s, you force one of the
btrfs raid1 copies to each of the mdraid0s, thus making allocation more
deterministic than btrfs raid10, and can thus again tolerate loss of two
devices, as long as they're from the same underlying mdraid0.
(Traditionally, raid1 on top of raid0 is called raid01, and is
discouraged compared to raid10, raid0 on top of raid1, because device
failure and replacement with the latter triggers a much more localized
rebuild than the former, across the pair of devices in the raid1 when
it's closest to the physical devices, across the whole array, one raid0
to the other, when the raid1 is on top. However, btrfs raid1's data
integrity and error repair from the good mirror feature is generally
considered to be useful enough to be worth the rebuild-inefficiency of
the raid01 design.)
So in regard to failure tolerance, btrfs raid10 is far closer to
traditional raid5, loss of a single device is tolerated, loss of a second
before a repair is complete generally means data loss -- there's not the
chance of it being on the same mirror set to save you that traditional
raid10 has.
Similarly, btrfs raid10 doesn't have the cleanly separate pair of mirrors
on raid0 arrays that traditional raid10 does, thus doesn't have the fault
tolerance of losing say the connection or power to one entire device
bank, as long as it's all one mirror set, that traditional raid10 has.
And again, doing the layered thing with btrfs raid1 on top and mdraid0
(or whatever else) underneath gets that back for you, if you set it up
that way, of course.
--
Duncan - List replies preferred. No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master." Richard Stallman
next prev parent reply other threads:[~2016-06-09 6:16 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-06-03 9:49 Recommended why to use btrfs for production? Martin
2016-06-03 9:53 ` Marc Haber
2016-06-03 9:57 ` Martin
2016-06-03 10:01 ` Hans van Kranenburg
2016-06-03 10:15 ` Martin
2016-06-03 12:55 ` Austin S. Hemmelgarn
2016-06-03 13:31 ` Martin
2016-06-03 13:47 ` Julian Taylor
2016-06-03 14:21 ` Austin S. Hemmelgarn
2016-06-03 14:39 ` Martin
2016-06-03 19:09 ` Christoph Anton Mitterer
2016-06-09 6:16 ` Duncan [this message]
2016-06-09 11:38 ` Austin S. Hemmelgarn
2016-06-09 17:39 ` Chris Murphy
2016-06-09 19:57 ` Duncan
2016-06-03 14:05 ` Chris Murphy
2016-06-03 14:11 ` Martin
2016-06-03 15:33 ` Austin S. Hemmelgarn
2016-06-04 0:48 ` Nicholas D Steeves
2016-06-04 1:48 ` Chris Murphy
2016-06-06 13:29 ` Austin S. Hemmelgarn
2016-06-04 1:34 ` Chris Murphy
2016-06-05 10:45 ` Mladen Milinkovic
2016-06-05 16:33 ` James Johnston
2016-06-05 18:20 ` Andrei Borzenkov
2016-06-06 1:47 ` Chris Murphy
2016-06-06 2:40 ` James Johnston
2016-06-06 13:36 ` Austin S. Hemmelgarn
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