Lo! You afaics want to configure the hdd as "writemostly". Google for
the term in combination with "linux" or "mdadm". It'll lead to
pages
like this:
http://tansi.info/hybrid/
http://hatim.eu/2014/05/25/leveraging-ssd-ephemeral-disks-in-ec2-part-2/
HTH, CU, knurd
P.S.: I considered not answering the question, because it afaics and
imho is off topic here (hence a answer only encourages people to send
more questions like this), as the question is not specific to Fedora in
any way. But this list is for "*Fedora kernel development*. This list is
used for discussion of changes to the Fedora kernel that aren't
necessarily relevant to the upstream kernel list. […]" (quote from
https://lists.fedoraproject.org/admin/lists/kernel.lists.fedoraproject.org/
) Hence I think you should send questions like that to a more general
Linux kernel mailing list in the future. But as I said: That just my option.
On 11.02.2017 15:16, Reindl Harald wrote:
> /dev/sda: Samsung SSD 850 EVO 2TB
> /dev/sdb: Samsung SSD 850 EVO 2TB
> /dev/sdc: ST2000DX002-2DV164
> /dev/sdd: ST2000DX002-2DV164
>
> normally i would expect that for heavy read-IO the two HDD drives would
> sleep and the two SSD working more or less like a RAID0 but as example
> running a btrfs-srub within a virtual machine on a 1.5 TB vdisk you can
> hear the rotating disks clearly and there are also unexpected lags while
> a disk IO auf around 700 MB/sec shows that it's basicly working
>
> pretty sure the lags are coming because read-access is also spread to
> HDD - why is that?
>
> /dev/md2:
> Version : 1.1
> Creation Time : Wed Jun 8 13:10:56 2011
> Raid Level : raid10
> Array Size : 3875222528 (3695.70 GiB 3968.23 GB)
> Used Dev Size : 1937611264 (1847.85 GiB 1984.11 GB)
> Raid Devices : 4
> Total Devices : 4
> Persistence : Superblock is persistent
>
> Intent Bitmap : Internal
>
> Update Time : Sat Feb 11 15:10:31 2017
> State : active
> Active Devices : 4
> Working Devices : 4
> Failed Devices : 0
> Spare Devices : 0
>
> Layout : near=2
> Chunk Size : 512K
>
> Name : localhost.localdomain:2 (local to host
> localhost.localdomain)
> UUID : ea253255:cb915401:f32794ad:ce0fe396
> Events : 1818029
>
> Number Major Minor RaidDevice State
> 4 8 35 0 active sync set-A writemostly
> /dev/sdc3
> 5 8 19 1 active sync set-B /dev/sdb3
> 7 8 51 2 active sync set-A writemostly
> /dev/sdd3
> 6 8 3 3 active sync set-B /dev/sda3
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