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Old 10-13-2010, 10:12 AM   #1
eagleamon
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Question Media server disk consumption and hdparm configuration


Hello everyone !

I have some general question with with quite an impact for the system I use
I have a media server with 2 raid1 mirrors of 2 1 To disk aggregated with lvm and used by a big 2 To ext3 partition.

A few days ago, munin (awesome!) started to send me notifications about smart attributes of some disks, and i'm not quite worried... :KS

Fyi, here's my hdparm.conf:

/dev/sda {
spindown_time = 54
}

/dev/sdb {
spindown_time = 54
}

/dev/sdc {
spindown_time = 54
}

/dev/sdd {
spindown_time = 54
}

So the question: regarding reliability and disk longevity, is it recommended to have them spin down while not used (most of the day, as these are storage unit for movies, pictures, documents..) or let them spin whatever.. ?

Those are WD green and according to Tom' Hardware, the consumption is only slightly greater in idle compared to standby.

The other question, (boat one as we say in french ) what is the best configuration here ? My 2 raid 1 stripes, or a raid 5 ? I know that two disks kill the raid 5, and they have to be of the same stripe to kill the lvm on the raid1, but what are the odds ?

I would really appreciate your opinion and knowledge on this!

Thanks in advance
 
Old 10-14-2010, 11:46 PM   #2
neonsignal
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Location: Melbourne, Australia
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Quote:
Originally Posted by eagleamon View Post
So the question: regarding reliability and disk longevity, is it recommended to have them spin down while not used (most of the day, as these are storage unit for movies, pictures, documents..) or let them spin whatever.. ?
There are longevity issues both ways: spinning up/down puts more stress on the drive motor than running it continuously, so you don't want to do it too often; but running the drive motor continuously will wear out the bearings.

So the question is not whether to have a spin down, but what length of time the spin down parameter should be. It probably should be in the minutes range (see man hdparm for details on what the value means, for example 120 = 600s).

As long as the spindown is something reasonable, then other factors will also play a part in longevity, such as protecting the drive from shocks and temperature extremes.

Quote:
what is the best configuration here ? My 2 raid 1 stripes, or a raid 5 ? I know that two disks kill the raid 5, and they have to be of the same stripe to kill the lvm on the raid1, but what are the odds ?
The advantages of different RAID configurations are complex, but you don't get something for nothing; it involves trade-offs.

I think you are comparing RAID 1+0 (or RAID 0+1) to a 4 disk RAID 5 here.

The RAID 0+1 will be more reliable, because as you say, the second disk failure is less likely to cause total failure. But it will also give you less storage than a 4 disk RAID 5, and will be slower.

A more equal comparison would be RAID 0+1 with a 4 disk RAID 6. Here the space efficiency is the same, and the speed comparable. But the RAID 6 will take 3 failures to cause total failure, so is more reliable.

Another consideration is that parity style RAID (such as RAID 5 and RAID 6) can be harder to rebuild than a straight mirror. This is not such an issue in a commercial environment, where you might have access to duplicate hardware, but in a home environment it is nice to be able to mount the drives after a failure occurred without rebuilding (eg you can mount the good striped pair as RAID 0).

You should also take care that you are not using the RAID mirror as a substitute for backups. It would be better in some circumstances to use the second pair of disks as offline backups, rather than an online mirror. The reason for this is that there are other failure modes: spikes through power supplies, accidental deletion of data, RAID controller failures, local incidents such as fire or flood, and so on, that can affect all online disks at the same time. The purpose of redundancy in RAID configurations is to provide continuity of access (server uptimes), rather than data backup.

Last edited by neonsignal; 10-14-2010 at 11:49 PM.
 
Old 10-15-2010, 05:18 AM   #3
eagleamon
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Registered: Oct 2010
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Thumbs up

First, thanks a lot for your complete answer !

For the spin down issue, I'm comforted and I'll maybe have to change the 54 value (4 min 30, to prevent the 5 min munin polling to prevent the drives from going to sleep). By the way, I guess it's not possible to get the disks temperature without waking them up ?

Regarding my storage configuration, indeed I use raid1 but with lvm instead of raid 0, I figured it would be easier to make it grow afterwards. As the idea is just to have one big partiton for medias (maybe a second for document in the future), is raid0 a smarter solution ? Will it be easy to add new raid1 stripes ?

Thanks again a lot !
 
Old 10-15-2010, 06:03 PM   #4
neonsignal
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Quote:
Originally Posted by eagleamon View Post
I use raid1 but with lvm instead of raid 0, I figured it would be easier to make it grow afterwards.
The LVM is a different level to the RAID. The reason to use RAID 0 is to speed access, because it means that data is streamed from both disks at the same time (thanks to the striping). The reason to use LVM is provide more flexibility in volume management.

So if the drives are fast enough for your application, then it is fine to stick with LVM, especially if you plan on changing volume sizes in future.

Last edited by neonsignal; 10-15-2010 at 06:05 PM.
 
  


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