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 it was superseded by RAID5.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