From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: RAID1 On 3 Drives Date: Sat, 20 Mar 2010 12:08:40 -0400 Message-ID: <4BA4F308.8010303@tmr.com> References: <4B9C11FB.7040604@gmx.net> <4B9C24B6.7040507@tmr.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: Carlos Mennens Cc: Mdadm List-Id: linux-raid.ids Carlos Mennens wrote: > On Sat, Mar 13, 2010 at 6:50 PM, Bill Davidsen wrote: > >> Having a spare on raid-1 is fairly pointless, it hurts performance and buys >> you nothing. Having one more copy of the data built and ready serves you >> better. >> > > Can you explain this as I find this interesting. How does having a > /boot partition on 3 drives with 1 spare hurt performance? Are you > saying that I would get better drive performance if I had all 4 disk > partitions active members of my RAID1 /boot? I just don't understand > how the 4th disk doing nothing but acting as a spare would hinder > performance. > > In RAID-1 any drive which is a member of the array is already a spare, they are all the same. So if a drive failed you would just stop using it. Having a spare makes no sense, since the spare would be rebuilt using the same data it would have contained as a member of the array. Note that while the "spare" is being rebuilt the array will be quite busy and response will be poor, not an issue if /boot is the only thing on the drives, but more of an issue if the array just uses partitions and the thrashing of a spare rebuild slows other arrays on the same drives. Finally, RAID-1 supposedly will read from an unbusy drive if there are multiple reads against the array, allowing overlapping of seek and transfer. That's only an issue for /boot if you are trying for a ten second boot time, but it applies to all RAID-1 arrays. > Secondly, if the above statement also applies to my / partition? Would > you suggest using all 4 drives as active partitions in a RAID5 array > too? If I have a 3 disk RAID5 and one hot spare, do you think I would > get less performance value for my configuration? > No, because the drives are not all the same, so you can't keep a copy of just the drive ready to fail next. For RAID-[456&10] you want at least one spare for reliability and fast return to full performance. Ask again if I didn't give enough detail. -- Bill Davidsen "We can't solve today's problems by using the same thinking we used in creating them." - Einstein