Date: Fri, 23 Jul 2010 15:34:31 +0200 From: Attila Nagy <bra@fsn.hu> To: Ivan Voras <ivoras@freebsd.org> Cc: freebsd-fs@freebsd.org Subject: Re: ZFS makes SSDs faster than memory! Message-ID: <4C499A67.9080707@fsn.hu> In-Reply-To: <i2c5fn$uhh$1@dough.gmane.org> References: <4C496EB0.7050004@fsn.hu> <i2c14p$g4f$1@dough.gmane.org> <20100723125051.GM53114@cicely7.cicely.de> <4C499733.5000104@fsn.hu> <i2c5fn$uhh$1@dough.gmane.org>
next in thread | previous in thread | raw e-mail | index | archive | help
On 07/23/10 15:29, Ivan Voras wrote: > On 07/23/10 15:20, Attila Nagy wrote: > > >> Maybe I should have written this first, but I'm not the only one reading >> from the machine. >> > You probably realize this makes all your performance data of suspicious > validity :) > Yes, this is the same I would write to this e-mail, but I can reproduce it. :) Fetching the same file not in the cache three times make the first the slowest, the second (after waiting a little to fall out of RAM) the fastest and the third the second fastest. I can consistently reproduce this behaviour, but only via network (ftpd/httpd) not from localhost. >> For random reads even the cheapest MLC outperforms a 7k2 SATA disk (only >> reads), and this is an Intel stuff, which can do 3000 RIOPS easily. >> >>> Are there any facts backup your assumption that data is really >>> read from memory, SSD, disk in the named cases? >>> E.g. by ARC/L2ARC and IO statistics. >>> >>> >> Yes. When downloading from L2ARC: >> L(q) ops/s r/s kBps ms/r w/s kBps ms/w %busy Name >> 0 174 174 21505 0.8 0 0 0.0 13.3| ad4 >> 0 169 169 21479 0.9 0 0 0.0 15.0| ad6 >> when downloading from ARC: >> L(q) ops/s r/s kBps ms/r w/s kBps ms/w %busy Name >> 0 26 19 1129 0.6 7 78 0.4 1.3| ad4 >> 0 19 12 1436 1.1 7 78 0.3 1.4| ad6 >> > So it looks like you encountered a problem where the memory-based ARC > cache read performance is incredibly bad? > I wouldn't call it incredibly bad, but it's worse than reading from L2ARC (2xSSD), which is pretty strange and not sane, at least to what I know about how things work in a computer. :)
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?4C499A67.9080707>