On 17 September, 2009 - Eugen Leitl sent me these 2,0K bytes: > On Wed, Sep 16, 2009 at 08:02:35PM +0300, Markus Kovero wrote: > > > It's possible to do 3-way (or more) mirrors too, so you may achieve better > > redundancy than raidz2/3 > > I understand there's almost no additional performance penalty to raidz3 > over raidz2 in terms of CPU load. Is that correct? > > So SSDs for ZIL/L2ARC don't bring that much when used with raidz2/raidz3, > if I write a lot, at least, and don't access the cache very much, according > to some recent posts on this list. > > How much drive space am I'm losing with mirrored pools versus raidz3? IIRC > in RAID 10 it's only 10% over RAID 6, which is why I went for RAID 10 in > my 14-drive SATA (WD RE4) setup.
It's not a fixed value per technology, it depends on the number of disks per group. RAID5/RAIDZ1 "loses" 1 disk worth to parity per group. RAID6/RAIDZ" loses 2 disks. RAIDZ3 loses 3 disks. Raid1/mirror loses half the disks. So in your 14 drive case, if you go for one big raid6/raidz2 setup (which is larger than recommended for performance reasons), you will lose 2 disks worth of storage to parity leaving 12 disks worth of data. With raid10 you will lose half, 7 disks to parity/redundancy. With two raidz2 sets, you will get (5+2)+(5+2), that is 5+5 disks worth of storage and 2+2 disks worth of redundancy. The actual redudancy/parity is spread over all disks, not like raid3 which has a dedicated parity disk. For more info, see for example http://en.wikipedia.org/wiki/RAID /Tomas -- Tomas Ögren, st...@acc.umu.se, http://www.acc.umu.se/~stric/ |- Student at Computing Science, University of Umeå `- Sysadmin at {cs,acc}.umu.se _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
