Re: Performance Testing MD-RAID10 with 1 failed drive

[Roger Heflin <rogerheflin@gmail.com>]

It is likely much simpler.

Using a 2 disks raid 1 array with 100 reads iops and 100 write iops to
the filesystem you would see this with 2 disks:   150 iops per disk
(100 writes + 50 reads), but with one disk only in the raid you see
200 iops/disk (100 reads+100writes) and at that 7200 rpm spinning
disks would be overcapacity. Now with 8 disks the numbers scale up,
but the general idea is still the same.  Once a disk fails then all of
the reads it was handling have to go to the single remaining disk and
that read load could result in that remaining disk not being able to
keep up.

The original poster needs to get sar or iostat stat to see what the
actual io rates are, but if they don't understand what the spinning
disk array can do fully redundant and with a disk failed it is not
unlikely that the IO load is higher than a can be sustained with a
single disk failed.

On Fri, Oct 21, 2022 at 6:34 AM Andy Smith <andy@strugglers.net> wrote:
>
> Hello,
>
> On Fri, Oct 21, 2022 at 10:15:42AM +0200, Pascal Hambourg wrote:
> > Le 21/10/2022 à 02:14, Andy Smith a écrit :
> > > Perhaps you could use dm-dust to make an unreliable block device
> > > from a real device?
> >
> > That seems uselessly complicated to me.
>
> Well I too do not understand why OP can't just fail one existing
> device, but it seemed important to them to experience actual errors
> and have it kicked out for that. A half way measure might be the
> offline / delete poking I mentioned in /sys/block.
>
> *shrug*
>
> Cheers,
> Andy
>
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