Re: [gentoo-user] mdadm and raid4

[Evgeny Bushkov <zhen@dotcomltd.ru>]

On 04.05.2011 16:45, Florian Philipp wrote:
> Am 04.05.2011 14:39, schrieb Florian Philipp:
>> Am 04.05.2011 11:08, schrieb Evgeny Bushkov:
>>> On 04.05.2011 11:54, Joost Roeleveld wrote:
>>>> On Wednesday 04 May 2011 10:07:58 Evgeny Bushkov wrote:
>>>>> On 04.05.2011 01:49, Florian Philipp wrote:
>>>>>> Am 03.05.2011 19:54, schrieb Evgeny Bushkov:
>>>>>>> Hi.
>>>>>>> How can I find out which is the parity disk in a RAID-4 soft array? I
>>>>>>> couldn't find that in the mdadm manual.  I know that RAID-4 features a
>>>>>>> dedicated parity disk that is usually the bottleneck of the array, so
>>>>>>> that disk must be as fast as possible. It seems useful to employ a few
>>>>>>> slow disks with a relatively fast disk in such a RAID-4 array.
>>>>>>>
>>>>>>> Best regards,
>>>>>>> Bushkov E.
>>>>>> You are seriously considering a RAID4? You know, there is a reason why
>>>>>> it was superseded by RAID5. Given the way RAID4 operates, a first guess
>>>>>> for finding the parity disk in a running array would be the one with the
>>>>>> worst SMART data. It is the parity disk that dies the soonest.
>>>>>>
>>>>>> From looking at the source code it seems like the last specified disk is
>>>>>> parity. Disclaimer: I'm no kernel hacker and I have only inspected the
>>>>>> code, not tried to understand the whole MD subsystem.
>>>>>>
>>>>>> Regards,
>>>>>> Florian Philipp
>>>>> Thank you for answering... The reason I consider RAID-4 is a few
>>>>> sata/150 drives  and a pair of sata II drives I've got. Let's look at
>>>>> the problem from the other side: I can create RAID-0(from sata II
>>>>> drives) and then add it to RAID-4 as the parity disk. It doesn't bother
>>>>> me if any disk from the RAID-0 fails, that wouldn't disrupt my RAID-4
>>>>> array. For example:
>>>>>
>>>>> mdadm --create /dev/md1 --level=4 -n 3 -c 128 /dev/sdb1 /dev/sdc1 missing
>>>>> mdadm --create /dev/md2 --level=0 -n 2 -c 128 /dev/sda1 /dev/sdd1
>>>>> mdadm /dev/md1 --add /dev/md2
>>>>>
>>>>> livecd ~ # cat /proc/mdstat
>>>>> Personalities : [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
>>>>> md2 : active raid0 sdd1[1] sda1[0]
>>>>>       20969472 blocks super 1.2 128k chunks
>>>>>
>>>>> md1 : active raid4 md2[3] sdc1[1] sdb1[0]
>>>>>       20969216 blocks super 1.2 level 4, 128k chunk, algorithm 0 [3/2] [UU_]
>>>>> [========>............]  recovery = 43.7% (4590464/10484608) finish=1.4min
>>>>> speed=69615K/sec
>>>>>
>>>>> That configuration works well, but I'm not sure if md1 is the parity
>>>>> disk here, that's why I asked. May be I'm wrong and RAID-5 is the only
>>>>> worth array, I'm just trying to consider all pros and cons here.
>>>>>
>>>>> Best regards,
>>>>> Bushkov E.
>>>> I only use RAID-0 (when I want performance and don't care about the data), 
>>>> RAID-1 (for data I can't afford to loose) and RAID-5 (data I would like to 
>>>> keep). I have never bothered with RAID-4.
>>>>
>> [...]
>>> I've run some tests with different chunk sizes, the fastest was
>>> raid-10(4 disks), raid-5(3 disks) was closely after. Raid-4(4 disks) was
>>> almost as fast as raid-5 so I don't see any sense to use it.
>>>
>>> Best regards,
>>> Bushkov E.
>>>
>>>
>>>
>> When you have an array with uneven disk speeds, you might consider using
>> the --write-mostly option of mdadm:
>> -W, --write-mostly
>>     subsequent devices lists in a --build, --create, or --add command
>> will be flagged as 'write-mostly'. This is valid for RAID1 only and
>> means that the 'md' driver will avoid reading from these devices if at
>> all possible. This can be useful if mirroring over a slow link.
>>
>> This should help in concurrent read and write operations because the
>> kernel will not dispatch read requests to a disk that is already having
>> trouble managing the write operations.
>>
>> On another point: Are you sure your disks have different speeds? SATA150
>> and 300 are no reliable indicator because most HDDs cannot saturate the
>> SATA port anyway. dd is still the most reliable way to measure
>> sequential throughput.
>>
>> Regards,
>> Florian Philipp
>>
> `man 4 md` also states that the "the last of the active devices in the
> array" is the parity disk in a RAID4.
>
> Regards,
> Florian Philipp
>
Thank you, that's what I was searching for. It seems I configured my
RAID-4 right. Nonetheless that array wasn't better than RAID-5. Finally
I created RAID-5 (two sataII drives and 1 sata/150) with lvm.
From a test "time dd if=/dev/zero of=/mnt/gentoo/bigfile bs=1M
count=3000" I've got 163 MB/s. That in any way better than my 1-disk
system(46.7 MB/s from the same test) that I've been using.
As regard interfaces speeds I ran hdparm command and got:
livecd src # hdparm -i /dev/sd[abd]|grep UDMA     
 UDMA modes: udma0 udma1 udma2 udma3 udma4 udma5 *udma6
 UDMA modes: udma0 udma1 udma2 udma3 udma4 udma5 *udma6
 UDMA modes: udma0 udma1 udma2 udma3 udma4 udma5 *udma6
So you are right, all the drives seem to utilize the same interface speed.

Best regards,
Bushkov E.