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Date:      Tue, 22 Jan 2013 10:18:52 -0800
From:      Matthew Ahrens <mahrens@delphix.com>
To:        Peter Jeremy <peter@rulingia.com>
Cc:        freebsd-fs <freebsd-fs@freebsd.org>, Wojciech Puchar <wojtek@wojtek.tensor.gdynia.pl>, FreeBSD Hackers <freebsd-hackers@freebsd.org>
Subject:   Re: ZFS regimen: scrub, scrub, scrub and scrub again.
Message-ID:  <CAJjvXiFXAwzy=hAABAHKzgfExtDVnYO-yi3H_JTqzTCm8Kg-cA@mail.gmail.com>
In-Reply-To: <20130122073641.GH30633@server.rulingia.com>
References:  <CACpH0Mf6sNb8JOsTzC%2BWSfQRB62%2BZn7VtzEnihEKmEV2aO2p%2Bw@mail.gmail.com> <alpine.BSF.2.00.1301211201570.9447@wojtek.tensor.gdynia.pl> <20130122073641.GH30633@server.rulingia.com>

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On Mon, Jan 21, 2013 at 11:36 PM, Peter Jeremy <peter@rulingia.com> wrote:
> On 2013-Jan-21 12:12:45 +0100, Wojciech Puchar <
wojtek@wojtek.tensor.gdynia.pl> wrote:
>>While RAID-Z is already a king of bad performance,
>
> I don't believe RAID-Z is any worse than RAID5.  Do you have any actual
> measurements to back up your claim?

Leaving aside anecdotal evidence (or actual measurements), RAID-Z is
fundamentally slower than RAID4/5 *for random reads*.

This is because RAID-Z spreads each block out over all disks, whereas RAID5
(as it is typically configured) puts each block on only one disk.  So to
read a block from RAID-Z, all data disks must be involved, vs. for RAID5
only one disk needs to have its head moved.

For other workloads (especially streaming reads/writes), there is no
fundamental difference, though of course implementation quality may vary.

>> Even better - use UFS.

To each their own.  As a ZFS developer, it should come as no surprise that
in my opinion and experience, the benefits of ZFS almost always outweigh
this downside.

--matt



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