Date: Mon, 13 Jul 2009 14:02:58 -0400 From: Maxim Khitrov <mkhitrov@gmail.com> To: mahlerrd@yahoo.com Cc: Free BSD Questions list <freebsd-questions@freebsd.org> Subject: Re: ZFS or UFS for 4TB hardware RAID6? Message-ID: <26ddd1750907131102y7a75c674n550c50a5af76da0d@mail.gmail.com> In-Reply-To: <78927.1741.qm@web51001.mail.re2.yahoo.com> References: <78927.1741.qm@web51001.mail.re2.yahoo.com>
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On Mon, Jul 13, 2009 at 1:46 PM, Richard Mahlerwein<mahlerrd@yahoo.com> wro= te: >> >> Your mileage may vary, but... >> >> I would investigate either using more spindles if you want >> to stick to RAID6, or perhaps using another RAID level if >> you will be with 4 drives for a while.=C2=A0 The reasoning >> is that there's an overhead with RAID 6 - parity blocks are >> written to 2 disks, so in a 4 drive combination you have 2 >> drives with data and 2 with parity. >> >> With 4 drives, you could get much, much higher performance >> out of RAID10 (which is alternatively called RAID0+1 or >> RAID1+0 depending on the manufacturer and on how accurate >> they wish to be, and on how they actually implemented it, >> too). This would also mean 2 usable drives, as well, so >> you'd have the same space available in RAID10 as your >> proposed RAID6. >> >> I would confirm you can, on the fly, convert from RAID10 to >> RAID6 after you add more drives.=C2=A0 If you can not, then >> by all means stick with RAID6 now! >> >> With 4 1 TB drives (for simpler examples) >> RAID5 =3D 3 TB available, 1 TB worth used in "parity". >> Fast reads, slow writes. >> RAID6 =3D 2 TB available, 2 TB worth used in "parity". >> Moderately fast reads, slow writes. >> RAID10 =3D 2 TB available, 2TB in duplicate copies (easier >> work than parity calculations).=C2=A0 Very fast reads, >> moderately fast writes. >> >> When you switch to, say, 8 drives, the numbers start to >> change a bit. >> RAID5 =3D 7TB available, 1 lost. >> RAID6 =3D 6TB available, 2 lost. >> RAID10 =3D 4TB available, 4 lost. >> > > Sorry, consider myself chastised for having missed the "Security is more = important than performance" bit. I tend toward solutions that show the most= value, and with 4 drives, it seems that I'd stick with the same "data secu= rity" only pick up the free speed of RAID10. =C2=A0Change when you get to 6= or more drives, if necessary. > > For data security, I can't answer for the UFS2 vs. ZFS. =C2=A0For hardwar= e setup, let me amend everything I said above with the following: > > Since you are seriously focusing on data integrity, ignore everything I s= aid but make sure you have good backups! =C2=A0:) > > Sorry, > -Rich No problem :) I've been doing some reading since I posted this question and it turns out that the controller will actually not allow me to create a RAID6 array using only 4 drives. 3ware followed the same reasoning as you; with 4 drives use RAID10. I know that you can migrate from one to the other when a 5th disk is added, but RAID10 can only handle 2 failed drives if they are from separate RAID1 groups. In this way, it is just slightly less resilient to failure than RAID6. With this new information, I think I may as well get one more 2TB drive and start with 6TB of RAID6 space. This will be less of a headache later on. - Max
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