Re: raid1 round-robin scheduler

[Heinz Mauelshagen <heinzm@redhat.com>]

On 03/11/2015 08:22 AM, konstantin wrote:
>
> 10.03.2015 17:22, Heinz Mauelshagen wrote:
>>
>> On 03/10/2015 12:55 PM, konstantin wrote:
>>>
>>>
>>> 19.02.2015 18:02, Heinz Mauelshagen пишет:
>>>>
>>>>
>>>> dm-mirror (i.e. "lvcreate --type mirror" or respective "dmsetup create
>>>> --table ...",
>>>> which is not the recommended raid1 layout any more) provides read
>>>> round-robin since long time.
>>>> You'd need an ancient kernel not to have it supported.
>>>>
>>>> "raid1"/"raid10" (the recommended targets) , i.e. the md-raid based
>>>> mappings accessible via the dm-raid target
>>>> do read optimizations as well. Use "lvcreate --type raid1/raid10 
>>>> ..." or
>>>> a respective dm table to set
>>>> those up. The former ("raid1") is the default in modern distributions
>>>> and configurable via setting
>>>> 'mirror_segtype_default = "raid1"' in /etc/lvm/lvm.conf.
>>>>
>>>> Heinz
>>>>
>>>>
>>>>
>>>>
>>>> On 02/19/2015 08:23 AM, konstantin wrote:
>>>>> What version of the kernel should I use to get a round-robin read
>>>>> implementation on LV raid1?
>>>>>
>>>>
>>>> -- 
>>>> dm-devel mailing list
>>>> dm-devel@redhat.com
>>>> https://www.redhat.com/mailman/listinfo/dm-devel
>>>
>>> I'm create raid1 lv with "lvcreate --type raid1 -m1 -L5G -n r1lv r1vg"
>>> on vg with two physical devices:
>>>
>>> lvs -a -o +devices
>>>   LV              VG   Attr     LSize Pool Origin Data%  Move Log
>>> Copy%  Convert Devices
>>>   r1lv            r1vg rwi-a-m- 5.00g 100.00
>>> r1lv_rimage_0(0),r1lv_rimage_1(0)
>>>   [r1lv_rimage_0] r1vg iwi-aor- 5.00g           /dev/sda(1)
>>>   [r1lv_rimage_1] r1vg iwi-aor- 5.00g           /dev/sdb(1)
>>>   [r1lv_rmeta_0]  r1vg ewi-aor- 4.00m           /dev/sda(0)
>>>   [r1lv_rmeta_1]  r1vg ewi-aor- 4.00m           /dev/sdb(0)
>>>
>>> but, reading is only one of the devices (i see nmon live disk
>>> utilization). There are solutions to ensure reading from the two 
>>> devices?
>>>
>>
>> How do you test?
>> Do you read from multiple threads?
>>
>>
>>
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>> dm-devel@redhat.com
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>
> really .. I used dd to test in one thread,

That's what I assumed.

> but why when I read in one thread I can not read from two PV devices 
> at the same time?
>

Your dd example causes streaming io which is what spindles can handle best.
Thus it would not make sense to split ios up.
For such kind of single-threaded streaming io with sensefull block size
a striped mapping would do better.

Try running dd/fio/... multiple times in parallel and you should see the 
expected effect.


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