From mboxrd@z Thu Jan 1 00:00:00 1970 From: Reindl Harald Subject: Re: RAID10 and 'writemostly' support Date: Sun, 19 Feb 2017 18:48:25 +0100 Message-ID: References: <07e8d394-2f95-caae-d971-a5fef009fefe@thelounge.net> <033aa14d-32a0-531d-7d7d-c01a0aeb13eb@youngman.org.uk> <7090c5c7-7f02-14a3-36f1-23b8bfe53043@turmel.org> <5a11be45-1793-d4b7-7cf5-3adaef5740d6@thelounge.net> <08f52436-9de4-76d0-99c0-c90dedee8ae1@turmel.org> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Return-path: In-Reply-To: <08f52436-9de4-76d0-99c0-c90dedee8ae1@turmel.org> Sender: linux-raid-owner@vger.kernel.org To: linux-raid@vger.kernel.org List-Id: linux-raid.ids Am 19.02.2017 um 18:31 schrieb Phil Turmel: > On 02/18/2017 06:35 PM, Reindl Harald wrote: >> >> Am 18.02.2017 um 23:20 schrieb Phil Turmel: > >>> If there are features (other than layouts) of raid10 that make you >>> prefer it to raid1, it would make sense to ask for those features to >>> be implemented in raid1. >> >> writemostly it's also very appealing on existing setups, the machine >> from where i type was installed in 2011 >> >> RAID1 don't have the benefit of doubled performance (also for writes, on >> a hybrid RAID slower but still faster than RAID1) *and* doubled space >> compared to a single disk combined with mirroring > > Doubled capacity? Vs. raid1? No. Raid10,n2 (,n2 is default) on two > devices yields the same capacity as raid1 on two devices. Unless I'm > misunderstanding your point. you are misunderstanding RAID1: 2x2 TB = 2 TB usable RAID10: 4x2 TB = 4 TB useable typically smaller disks are cheaper and when i installed the 4x2 TB RAID10 4 TB disks where not that common and 4 TB SSD not available at all (and 2 TB SSD unpaibale) >> another example: on machines like a HP microserver with only 4 drive >> slots that you could easily improve read-performance which is for many >> workloads the most important part by just switch half of the disk to SSD >> >> price calculation for a hybrid RAID10 with 10 disks: >> 5x4 TB SSD = 5 x 1400€ = 7000€ >> 5x4 TB HDD = 5 x 100€ = 500€ >> total price 7500€ versus 14000€ for flash-only > > What is preventing you from using the existing raid1 in pairs with > write mostly, then layering raid0 on top of them for the capacity you > are trying to achieve? No new code required. What you are asking for > really is raid1+0, which MD raid allows you to assemble yourself. already existing setups and the easier configuraion of RAID10 than wrap 2 RAID1 into a RAID0 especially at inital setup time when you also cover the os setup itself /dev/md0 ext4 485M 33M 448M 7% /boot /dev/md1 ext4 29G 6,8G 22G 24% / /dev/md2 ext4 3,6T 2,3T 1,4T 63% /mnt/data md0: RAID1 md1: RAID10 md2: RAID10 it's really not funny to change that existing layout from RAID10 to RAID0+RAID1 >> i would be *seriously* willing to pay the inital patch for any kernel >> maintainer who takes it over - Fedora regulary does kernel-rebases on GA >> versions > > Since no new kernel code is needed to achieve what you desire, I doubt > a kernel patch for it would be accepted. (But I'm not a maintainer, so > YMMV.) This is really a user-space question, along the lines of > "should/could mdadm automate creation of dual layers like raid1+0?" at least "mdadm" in the current state should just refuse "--write-mostly" when the array is a RAID10 - in that case i would have known by testing it based on http://www.tansi.org/hybrid/ in a virtual machine that it *really* don't work with RAID10 obviously there is code needed to achieve "writemostly" on the most common setup of 4 disks for a RAID10 where you later try to replace half of the disks with SSD and have writes only on the remaining HDD there are so many workloads where read-performance is more imprtant (boot, start of large applications, start virtual machines, rsync large data...)