From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: Write-intent bitmap decreases or increase performance of RAID5? Date: Sun, 04 Jul 2010 20:17:10 -0400 Message-ID: <4C312486.3040403@tmr.com> References: <20100629201153.GA13018@localhost> <20100630111854.006ec4d0@natsu> <20100701064044.GB6058@localhost> <20100702024927.21c4f814@natsu> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20100702024927.21c4f814@natsu> Sender: linux-raid-owner@vger.kernel.org To: Roman Mamedov Cc: Shaochun Wang , "linux-raid@vger.kernel.org" List-Id: linux-raid.ids Roman Mamedov wrote: > On Thu, 1 Jul 2010 14:40:44 +0800 > Shaochun Wang wrote: > > >> -bash-4.1$ sudo dd if=/dev/zero of=test.dd bs=1M count=5000 >> conv=fdatasync,notrunc Password: >> 5000+0 records in >> 5000+0 records out >> 5242880000 bytes (5.2 GB) copied, 63.497 s, 82.6 MB/s >> >> -bash-4.1$ sudo dd if=/dev/zero of=test.dd bs=1M count=5000 >> conv=fdatasync,notrunc 5000+0 records in >> 5000+0 records out >> 5242880000 bytes (5.2 GB) copied, 18.1033 s, 290 MB/s >> >> I don't know why the second dd becomes 290MB/s and the first 82.6MB/s. >> > > That's because the first time the filesystem had to increase the file's size > 5000 times by allocating additional 1 MB, and the second time it was just > writing to an already allocated file. If you see such a big difference here, > run that test 3 or more times, and discard the first run's results. > > Unless the notrunc option is used, the file is truncated and allocated all over again. I'm pretty sure the inode is reused, that's down in the noise. -- Bill Davidsen "We can't solve today's problems by using the same thinking we used in creating them." - Einstein