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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