On Sun, Nov 05, 2006 at 02:12:58AM +0100, Robert Milkowski wrote: > Hello Robert, > > Thursday, November 2, 2006, 5:12:37 PM, you wrote: > > RM> Hello zfs-discuss, > > > RM> Server: x4500, 2x Opetron 285 (dual-core), 16GB RAM, 48x500GB > > RM> filebench/randomread script, filesize=256GB > > RM> 2 disks for system, 2 disks as hot-spares, atime set to off for a > RM> pool, cache_bshift set to 8K (2^13), recordsize untouched (default). > > RM> pool: 4x raid-z (5 disks) + 4x raid-z (6 disks) means that one pool > RM> was created wit 4 raid-z1 groups each with 5 disks and another 4 > RM> raid-z1 groups each with 6 disks. > > > RM> 1. pool: 4x raid-z (5 disks) + 4x raid-z (6 disks) > RM> (36 disks of usable space) > > RM> a. nthreads = 1 ~60 ops > RM> b. nthreads = 4 ~250 ops > RM> c. nthreads = 8 ~520 ops > RM> d. nthreads = 128 ~1340 ops > > RM> 1340/8 = 167 ops > RM> > > Now the same pool config but actual RAID-5 is done using SVM and zfs > just does striping between SVM R5 devices. > > with nthreads=128 I get ~3680 ops > > which is almost 3x as much as with raid-z. > > I don't like this config but maybe it's a better way to go than with > raid-z after all - at least with some environments. > > > ps. however creating large file is about 4x slower than on raid-z
In my opinion RAID-Z is closer to RAID-3 than to RAID-5. In RAID-3 you
do only full stripe writes/reads, which is also the case for RAID-Z.
What I found while working on RAID-3 implementation for FreeBSD was that
for small RAID-3 arrays there is a way to speed up random reads up to
40% by using parity component in a round-robin fashion. For example
(DiskP stands for partity component):
Disk0 Disk1 Disk2 Disk3 DiskP
And now when I get read request I do:
Request number Components
0 Disk0+Disk1+Disk2+Disk3
1 Disk1+Disk2+Disk3+(Disk1^Disk2^Disk3^DiskP)
2 Disk2+Disk3+(Disk2^Disk3^DiskP^Disk0)+Disk0
3 Disk3+(Disk3^DiskP^Disk0+Disk1)+Disk0+Disk1
etc.
+ - concatenation
^ - XOR
In other words for every read request different component is skipped.
It was still a bit slower than RAID-5, though. And of course writes in
RAID-3 (and probably for RAID-Z) are much, much faster.
--
Pawel Jakub Dawidek http://www.wheel.pl
[EMAIL PROTECTED] http://www.FreeBSD.org
FreeBSD committer Am I Evil? Yes, I Am!
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